Śrī Kṛṣṇa Himself has enumerated the basic principles of a caste system that is real and universal. The four social orders (intellectual, administrative, mercantile, and laborer) are set by Him according to the qualities these persons have acquired through their actions under the modes of nature. So although in one sense He is the maker of this castesystem all over the world, still, in another sense, He is to be understood as not its maker. That is, He is not the maker of a tyrannical and unnatural caste system in which the faithless dictate one's position according to one's birth. Rather, He is the maker of a caste system that is applicable universally, is voluntary and natural, and is based on one's qualities and abilities.
(Message of Godhead).

The Lord says that the institution of four varṇas and four āśramas "is created by Me." Anything created by the Lord cannot be closed or covered. The divisions of varṇas and āśramas will continue to exist, either in their original form or in degraded form, but because they are created by the Lord, the Supreme Personality of Godhead, they cannot be extinguished. They are like the sun, a creation of God, and therefore will remain. Either covered by clouds or in a clear sky, the sun will continue to exist. Similarly, when the varṇāśrama system becomes degraded, it appears as a hereditary caste system, but in every society there is an intelligent class of men, a martial class, a mercantile class and a laborer class. When they are regulated for cooperation among communities according to the Vedic principles, then there is peace and spiritual advancement. But when there is hatred and malpractice and mutual mistrust in the caste system, the whole system becomes degraded, and as stated herein, it creates a deplorable state. At the present moment, the entire world is in this deplorable condition because of giving rights to so many interests. This is due to the degradation of the four castes of varṇas and āśramas.
(Srimad Bhagavatam------3:21:52-54------purport).

The qualifications of the various orders of the caste system are enumerated in Bhagavad-gītā, and here we shall touch on them briefly. The brāhmaṇas are the highest social order, and they imbibe the modes of goodness and are engaged in the activities of equality, restraint, and forgiveness. The kṣatriyas are the second-highest social order, and they imbibe the qualities of creative passion and are engaged in the activities of public leadership as executive heads of different political and social bodies. The vaiśyas are the third social order. They imbibe mixed qualities, namely creative passion as well as the darkness of ignorance, and generally they are engaged as farmers and merchants. The śūdras are the lowest social order, inasmuch as they imbibe the modes of darkness, or ignorance, and generally take up the service of the other three social orders. As a class, the śūdras are servitors of the whole mundane social body. In the present age of darkness, which is known as the Kali-yuga, the age of quarrel, hypocrisy, and ignorance, virtually everyone is born a śūdra.
(Message of Godhead).

According to Vedic culture, anyone who does not follow the regulative principles observed by the higher castes (the brāhmaṇas, kṣatriyas and vaiśyas) or even those observed by the laborer class (the śūdras) is called a mleccha or yavana. The Vedic social situation is so planned that persons accepted as śūdras can gradually be elevated to the position of brāhmaṇas by the cultural advancement known as saṁskāra, or the purificatory process. The verdict of the Vedic scriptures is that no one becomes a brāhmaṇa or a mleccha simply by birth; by birth everyone is accepted as a śūdra. One has to elevate himself by the purificatory process to the stage of brahminical life. If he doesn’t, if he degrades himself further, he is then called a mleccha or yavana. Kālayavana belonged to the class of mlecchas and yavanas. Contaminated by sinful activities, he could not approach Kṛṣṇa.
(Krsna Book).

According to the Vedic system there are four castes—the brāhmaṇas, kṣatriyas, vaiśyas and śūdras—and below them are the pañcamas (literally, "members of the fifth group"), who are lower than the śūdras. The higher castes—the brāhmaṇas, the kṣatriyas and even the vaiśyas—were known as brāhmaṇa-saj-jana. The brāhmaṇas especially were known as saj-jana, or respectable gentlemen who guided the entire society. If there were disputes in the village, people would approach these respectable brāhmaṇas to settle them. Now it is very difficult to find such brāhmaṇas and saj-janas, and thus every village and town is so disrupted that there is no peace and happiness anywhere. To revive a fully cultured civilization, the scientific division of society into brāhmaṇas, kṣatriyas, vaiśyas and śūdras must be introduced all over the world. Unless some people are trained as brāhmaṇas, there cannot be peace in human society.
(Sri Caitanya Caritamrta------1:17:42------purport).

After enjoying fully amongst themselves, the queens and Lord Kṛṣṇa would come out of the water, and they would give up their wet garments, which were very valuable, to be taken away by the professional singers and dancers. These singers and dancers had no means of subsistence other than the rewards of valuable garments and ornaments left by the queens and kings on such occasions. The whole system of society was so well planned that all the members of society in their different positions as brāhmaṇas, kṣatriyas, vaiśyas and śūdras had no difficulty in earning their livelihood. There was no competition among the divisions of society. The original conception of the caste system was so planned that one group of men engaged in a particular type of occupation would not compete with another group of men engaged in a different occupation.
(Krsna Book).

The two misconceptions of life, namely "I" and "mine," are verily manifested in two classes of men. In the lower state the conception of "mine" is very prominent, and in the higher state the misconception of "I" is prominent. In the animal state of life the misconception of "mine" is perceivable even in the category of cats and dogs, who fight with one another with the same misconception of "mine." In the lower stage of human life the same misconception is also prominent in the shape of "It is my body," "It is my house," "It is my family," "It is my caste," "It is my nation," "It is my country," and so on. And in the higher stage of speculative knowledge, the same misconception of "mine" is transformed into "I am," or "It is all I am," etc. There are many classes of men comprehending the same misconception of "I" and "mine', in different colors. But the real significance of "I" can be realized only when one is situated in the consciousness of "I am the eternal servitor of the Lord." This is pure consciousness, and the whole Vedic literatures teach us this conception of life.
(Srimad Bhagavatam------2:9:3------purport).

The demons misinterpret the words of Lord Kṛṣṇa concerning caste and social division, and they act capriciously on that basis. But this cannot blemish Lord Kṛṣṇa or His words. In the Bhagavad-gītā (4.13) Lord Kṛṣṇa clearly says,
According to the three modes of material nature and the work associated with them, the four divisions of human society are created by Me. And, although I am the creator of this system, you should know that I am yet the nondoer, being unchangeable.
The four divisions of society—namely intellectuals, administrators, merchants, and laborers—should be determined not by birth but by merit, just as one becomes a doctor or a judge not by birthright but by merit alone. In this world of the three modes of material nature, social classes have always existed. Therefore a person's birth should never determine his caste or class in society. The four classes were created according to a person's qualifications.
(Renunciation through Wisdom).

Human society all over the world is divided into four castes and four orders of life. The four castes are the intelligent caste, the martial caste, the productive caste and the laborer caste. These castes are classified in terms of one's work and qualification and not by birth. Then again there are four orders of life, namely the student life, the householder's life, the retired and the devotional life. In the best interest of human society there must be such divisions of life, otherwise no social institution can grow in a healthy state. And in each and every one of the abovementioned divisions of life, the aim must be to please the supreme authority of the Personality of Godhead. This institutional function of human society is known as the system of varṇāśrama-dharma, which is quite natural for the civilized life. The varṇāśrama institution is constructed to enable one to realize the Absolute Truth.
(Srimad Bhagavatam------1:2:13------purport).

A son born to a doctor is not necessarily sure to grow up to be a doctor; similarly, the progeny of the four classes of society do not automatically fix their future career according to that of their parents. The scriptures describe in detail the divisions of society, with their inherent characteristics. Therefore we commit a serious mistake when we regard the different classes of men as belonging to particular countries or races. The Indian culture of today is restricted by the hereditary caste system and kept in the custody of narrow-minded people who are like frogs in a well. If instead India had spread the transcendental message of Bhagavad-gītā in the generous manner befitting a noble brāhmaṇa, then peace and tranquillity in this world would not be in such acutely short supply. By the propagation of brahminical culture, the world would have greatly prospered. Instead, the Vedic culture has been seriously maimed by the imposition of the hereditary caste system, and this has had grievously adverse effects on the world. The Supreme Lord in His incarnation as Lord Caitanya has opened many avenues to peaceful living by propagating the brahminical culture, which He calls the religion of the soul. Those who are fortunate can emulate His life, follow His divine teachings, and perfect their lives.
(Renunciation through Wisdom).

There is nothing in the codes of Śrī Kṛṣṇa to stipulate that these devoted persons will make their appearance within the boundaries of a particular caste, creed, color, or country. These devoted persons can and do appear everywhere, without any restriction of caste, creed, color, or country. So everyone, whatever and whoever he may be, is eligible to be a devotee of Śrī Kṛṣṇa. To confirm this fact, in Bhagavad-gītā the Personality of Godhead says the following words: "O son of Pṛthā, even those who are faithless and are of lower birth—including fallen women or professional prostitutes, ignorant manual laborers, and the merchant class—all shall attain perfection and reach the Kingdom of God, if they actually take shelter of the devotional service of the Personality of Godhead, Śrī Kṛṣṇa." In other words, the unscrupulous caste system now dominant in the society of the asuras or the faithless cannot be any barrier to approaching Śrī Kṛṣṇa, the Absolute Personality of Godhead.
(Message of Godhead).

The principal sign of the age of Kali is that lower-caste śūdras, i.e., men without brahminical culture and spiritual initiation, will be dressed like administrators or kings, and the principal business of such non-kṣatriya rulers will be to kill the innocent animals, especially the cows and the bulls, who shall be unprotected by their masters, the bona fide vaiśyas, the mercantile community. In the Bhagavad-gītā (18.44), it is said that the vaiśyas are meant to deal in agriculture, cow protection and trade. In the age of Kali, the degraded vaiśyas, the mercantile men, are engaged in supplying cows to slaughterhouses. The kṣatriyas are meant to protect the citizens of the state, whereas the vaiśyas are meant to protect the cows and bulls and utilize them to produce grains and milk. The cow is meant to deliver milk, and the bull is meant to produce grains. But in the age of Kali, the śūdra class of men are in the posts of administrators, and the cows and bulls, or the mothers and the fathers, unprotected by the vaiśyas, are subjected to the slaughterhouses organized by the śūdra administrators.
(Srimad Bhagavatam------1:17:1------purport).

If we examine human affairs in the light of the caste system as created by the Personality of Godhead, surely we can visualize the four social orders functioning in every part of the world. In every part of the globe, wherever there is human habitation, there are some persons who have the qualifications of brāhmaṇas, and there are others who have the qualifications of kṣatriyas, vaiśyas, and śūdras. The various modes of nature are persistent in every corner of the universe, and since brāhmaṇas, kṣatriyas, and so forth are simply products of the modes of nature, how can one say that the four castes do not exist in a particular part of the world? This is absurd. In every country and at all times there have been, there are, and there will be the four social orders, according to the modes of nature.
(Message of Godhead).

Although Haridāsa Ṭhākura was such a highly exalted Vaiṣṇava that he was addressed as Haridāsa Gosvāmī, he still did not like to disturb the common sense of the general populace. Haridāsa Ṭhākura was so exalted that he was addressed as ṭhākura and gosāñi, and these titles are offered to the most advanced Vaiṣṇavas. The spiritual master is generally called gosāñi, and ṭhākura is used to address the paramahaṁsas, those in the topmost rank of spirituality. Nonetheless, Haridāsa Ṭhākura did not want to go near the temple, although he was called there by Śrī Caitanya Mahāprabhu Himself. The Jagannātha temple still accepts only those Hindus who are in the varṇāśrama order. Other castes, especially those who are not Hindu, are not allowed to enter the temple. This is a long-standing regulation, and thus Haridāsa Ṭhākura, although certainly competent and qualified to enter the temple, did not want even to go near it. This is called Vaiṣṇava humility.
(Sri Caitanya Caritamrta------2:11:165------purport).

A pure devotee cuts off the limited ties of affection for his family and widens his activities of devotional service for all forgotten souls. The typical example is the band of six Gosvāmīs, who followed the path of Lord Caitanya. All of them belonged to the most enlightened and cultured rich families of the higher castes, but for the benefit of the mass of population they left their comfortable homes and became mendicants. To cut off all family affection means to broaden the field of activities. Without doing this, no one can be qualified as a brāhmaṇa, a king, a public leader or a devotee of the Lord. The Personality of Godhead, as an ideal king, showed this by example. Śrī Rāmacandra cut off the tie of affection for His beloved wife to manifest the qualities of an ideal king.
(Srimad Bhagavatam------1:8:41------purport).

Generally brāhmaṇas are puffed up with false prestige because they belong to the aristocracy and perform many Vedic sacrifices. In South India especially, this fastidious position is most prominent. At any rate, this was the case five hundred years ago. Śrī Caitanya Mahāprabhu actually started a revolution against this brahminical system by inaugurating the chanting of the Hare Kṛṣṇa mantra. By this chanting, one can be delivered regardless of caste, creed, color or social position. Whoever chants the Hare Kṛṣṇa mahā-mantra is immediately purified due to the transcendental position of devotional service. Śrī Caitanya Mahāprabhu is here hinting to Vallabha Bhaṭṭācārya that an exalted brāhmaṇa who makes sacrifices and follows Vedic principles should not neglect a person who is engaged in devotional service by chanting the holy name of the Lord.
(Sri Caitanya Caritamrta------2:19:69------purport).

The brāhmaṇas are considered to be the head and brains of the social body, and the kṣatriyas are considered to be the arms of the social body. The arms are required to protect the body from all harm, but the arms must act according to the directions of the head and brain. That is a natural arrangement made by the supreme order, for it is confirmed in the Bhagavad-gītā that four social orders or castes, namely the brāhmaṇas, the kṣatriyas, the vaiśyas and the śūdras, are set up according to quality and work done by them. Naturally the son of a brāhmaṇa has a good chance to become a brāhmaṇa by the direction of his qualified father, as a son of a medical practitioner has a very good chance to become a qualified medical practitioner. So the caste system is quite scientific. The son must take advantage of the father's qualification and thus become a brāhmaṇa or medical practitioner, and not otherwise. Without being qualified, one cannot become a brāhmaṇa or medical practitioner, and that is the verdict of all scriptures and social orders. Herein Śṛṅgi, a qualified son of a great brāhmaṇa, attained the required brahminical power both by birth and by training, but he was lacking in culture because he was an inexperienced boy.
(Srimad Bhagavatam------1:8:33------purport).

Anyone from any part of the world who becomes well conversant in the science of Kṛṣṇa becomes a pure devotee and a spiritual master for the general mass of people and may reclaim them by purification of heart. Though a person be even the most sinful man, he can at once be purified by systematic contact with a pure Vaiṣṇava. A Vaiṣṇava, therefore, can accept a bona fide disciple from any part of the world without any consideration of caste and creed and promote him by regulative principles to the status of a pure Vaiṣṇava who is transcendental to brahminical culture. The system of caste, or varṇāśrama-dharma, is no longer regular even amongst the so-called followers of the system. Nor is it now possible to reestablish the institutional function in the present context of social, political and economic revolution. Without any reference to the particular custom of a country, one can be accepted to the Vaiṣṇava cult spiritually, and there is no hindrance in the transcendental process. So by the order of Lord Śrī Caitanya Mahāprabhu, the cult of Śrīmad-Bhāgavatam or the Bhagavad-gītā can be preached all over the world, reclaiming all persons willing to accept the transcendental cult. Such cultural propaganda by the devotees will certainly be accepted by all persons who are reasonable and inquisitive, without any particular bias for the custom of the country.
(Srimad Bhagavatam------2:4:18------purport).

Although individual souls are all different, the Superself, or Supersoul, is the Supreme Personality of Godhead. Individually one's self may differ from others in certain qualities and may engage in different activities, such as those of a brāhmaṇa, kṣatriya or vaiśya, but when there is complete cooperation among different individual souls, the Supreme Personality of Godhead as Supersoul, Paramātmā, being one in every individual soul, is pleased and gives them all protection. As stated before, the brāhmaṇas are produced from the mouth of the Lord, and the kṣatriyas are produced from the chest or arms of the Lord. If the different castes or social sections, although apparently differently occupied in different activities, nevertheless act in full cooperation, then the Lord is pleased. This is the idea of the institution of four varṇas and four āśramas. If the members of different āśramas and varṇas cooperate fully in Kṛṣṇa consciousness, then society is well protected by the Lord, without doubt.
(Srimad Bhagavatam------3:22:4------purport).

"Just by pouring water on the root of a tree, one nourishes its trunk and all of its branches, fruits and flowers, and just by supplying food to the stomach, one satisfies all the limbs of the body. Similarly, by worshiping Lord Viṣṇu one can satisfy everyone." Kṛṣṇa consciousness is not a sectarian religious movement. Rather, it is meant for all-embracing welfare activities for the world. One can enter this movement without discrimination in terms of caste, creed, religion or nationality. If one is trained to worship the Supreme Personality of Godhead, Kṛṣṇa, who is the origin of viṣṇu-tattva, one can become fully satisfied and perfect in all respects.
(Srimad Bhagavatam------8:5:49------purport).