We learn from the Śrīmad-Bhāgavatam that, due to māyā, which makes the living entity fall down from spiritual practice, certain obnoxious atheists try hard to create a smokescreen of philosophical jargon around the Supreme Lord to keep Him hidden from the general populace. The result of this effort is also described in the Bhāgavatam (12.3.43):
kalau na rājan jagatāṁ paraṁ guruṁ
tri-loka-nāthānata-pāda-paṅkajam
prāyeṇa martyā bhagavantam acyutaṁ
yakṣyanti pāṣaṇḍa-vibhinna-cetasaḥ
O King! In the Age of Kali people's intelligence will be diverted by atheism, and they will almost never offer sacrifice to the Supreme Personality of Godhead, who is the supreme spiritual master of the universe. Although the great personalities who control the three worlds all bow down to the lotus feet of the Supreme Lord, the petty and miserable human beings of this age will not do so.
(Renunciation through Wisdom).

The cycle of four yugas, or millenniums—namely, Satya, Tretā, Dvāpara, and Kali—goes around a thousand times in one day of Lord Brahmā. The Bhagavad-gītā (8.17) confirms this: sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ. "By human calculation, a thousand ages taken together form the duration of Brahmā's one day." According to the Vedic calculation, one day of Brahmā sees the coming and going of fourteen Manus. Therefore, each Manu lives for seventy-one cycles of the four millenniums. At present we are in the period of Vaivasvata Manu, in the twenty-eighth cycle of the four millenniums, and it is the Kali-yuga. This Kali-yuga is very special, however, because Lord Caitanya appears in this age in His original form and propagates the esoteric science of pure love of Godhead. All this we learn from the scriptures. We have great expectations that this science of pure love of Godhead will be propagated world-wide in the immediate future.
(Renunciation through Wisdom).

During the Satya-yuga the mode of goodness is in abundance. Or one can say that when the quality of goodness increases in a person to the extent that he becomes situated in his original constitutional identity as a servant of the Lord, thus making his human life a complete success, at that time he enjoys the bliss and tranquillity of the Satya-yuga. The three modes—goodness, passion, and ignorance—are always present in this material nature. According to the predominance of a particular mode, the yugas change from Satya to Tretā to Dvāpara to Kali. The jīvas in Kali-yuga are predominantly in the mode of ignorance, and with with the increase of this mode the threefold material miseries expand unlimitedly. Thus people today are afflicted by a short life-span, ill luck, warped intelligence, lethargy, disease, and many other sufferings.
(Renunciation through Wisdom).

As there are seasonal changes within a year, so there are changing ages in the duration of the manifest cosmic world. These changing ages are called yugas, or periods. As there are three modes of nature, there are also various ages dominated by these three modes. The period dominated by the mode of goodness is called Satya-yuga, the period of passion is called Tretā-yuga, the period of mixed passion and ignorance is called Dvāpara-yuga, and the period of darkness and ignorance (the last period) is called Kali-yuga, or the age of quarrel. The word kali means "quarrel." Kali-yuga is compared to the rainy season because many difficulties in life are experienced during this damp season.
(Light of the Bhagavata).

In this age of Kali, when a slight difference of opinion leads to quarrel, even to the extent of riots, it is the duty of the intelligent men, the brāhmaṇas, to selflessly inspire the richer people to sacrifice for this purpose. It is suggested herewith that the men of the intelligent class should not themselves try to become kṣatriyas or vaiśyas, nor should they engage themselves in the occupations of the various other classes; rather, the brāhmaṇas should simply guide them in spiritual cultivation, just as the wind carries the clouds to other places to pour water. The wind itself does not take up the responsibility for pouring water.
(Light of the Bhagavata).

This age of Kali is not at all suitable for self-realization as was Satya-yuga, the golden age, or Tretā- or Dvāpara-yugas, the silver and copper ages. For self-realization, the people in Satya-yuga, living a lifetime of a hundred thousand years, were able to perform prolonged meditation. And in Tretā-yuga, when the duration of life was ten thousand years, self-realization was attained by performance of great sacrifice. And in the Dvāpara-yuga, when the duration of life was one thousand years, self-realization was attained by worship of the Lord. But in the Kali-yuga, the maximum duration of life being one hundred years only and that combined with various difficulties, the recommended process of self-realization is that of hearing and chanting of the holy name, fame, and pastimes of the Lord.
(Srimad Bhagavatam-----1:1:21-----purport).

Lord Buddha, a powerful incarnation of the Personality of Godhead, appeared in the province of Gayā (Bihar) as the son of Añjanā, and he preached his own conception of nonviolence and deprecated even the animal sacrifices sanctioned in the Vedas. At the time when Lord Buddha appeared, the people in general were atheistic and preferred animal flesh to anything else. On the plea of Vedic sacrifice, every place was practically turned into a slaughterhouse, and animal-killing was indulged in unrestrictedly. Lord Buddha preached nonviolence, taking pity on the poor animals. He preached that he did not believe in the tenets of the Vedas and stressed the adverse psychological effects incurred by animal-killing. Less intelligent men of the age of Kali, who had no faith in God, followed his principle, and for the time being they were trained in moral discipline and nonviolence, the preliminary steps for proceeding further on the path of God realization.
(Srimad Bhagavatam-----1:3:24-----purport).

In the glorious days, or before the advent of the age of Kali, the brāhmaṇas, the cows, the women, the children and the old men were properly given protection.
1. The protection of the brāhmaṇas maintains the institution of varṇa and āśrama, the most scientific culture for attainment of spiritual life.
2. The protection of cows maintains the most miraculous form of food, i.e., milk for maintaining the finer tissues of the brain for understanding higher aims of life.
3. The protection of women maintains the chastity of society, by which we can get a good generation for peace, tranquillity and progress of life.
4. The protection of children gives the human form of life its best chance to prepare the way of liberty from material bondage. Such protection of children begins from the very day of begetting a child by the purificatory process of garbhādhāna-saṁskāra, the beginning of pure life.
5. The protection of the old men gives them a chance to prepare themselves for better life after death.
This complete outlook is based on factors leading to successful humanity as against the civilization of polished cats and dogs. The killing of the above-mentioned innocent creatures is totally forbidden because even by insulting them one loses one's duration of life. In the age of Kali they are not properly protected, and therefore the duration of life of the present generation has shortened considerably.
(Srimad Bhagavatam-----1:8:5-----purport).

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).

The next symptom of the age of Kali is that principles of religion, which are all spotlessly white, like the white lotus flower, will be attacked by the uncultured śūdra population of the age. They may be descendants of brāhmaṇa or kṣatriya forefathers, but in the age of Kali, for want of sufficient education and culture of Vedic wisdom, such a śūdra-like population will defy the principles of religion, and persons who are religiously endowed will be terrified by such men. They will declare themselves as adherents of no religious principles, and many "isms" and cults will spring up in Kali-yuga only to kill the spotless bull of religion.
(Srimad Bhagavatam-----1:17:2-----purport).

The next symptom of the age of Kali is the distressed condition of the cow. Milking the cow means drawing the principles of religion in a liquid form. The great ṛṣis and munis would live only on milk. Śrīla Śukadeva Gosvāmī would go to a householder while he was milking a cow, and he would simply take a little quantity of it for subsistence. Even fifty years ago, no one would deprive a sādhu of a quart or two of milk, and every householder would give milk like water. For a Sanātanist (a follower of Vedic principles) it is the duty of every householder to have cows and bulls as household paraphernalia, not only for drinking milk, but also for deriving religious principles. The Sanātanist worships cows on religious principles and respects brāhmaṇas. The cow's milk is required for the sacrificial fire, and by performing sacrifices the householder can be happy. The cow's calf not only is beautiful to look at, but also gives satisfaction to the cow, and so she delivers as much milk as possible. But in the Kali-yuga, the calves are separated from the cows as early as possible for purposes which may not be mentioned in these pages of Śrīmad-Bhāgavatam.
(Srimad Bhagavatam-----1:17:3-----purport).

In the age of Kali, the poor helpless animals, especially the cows, which are meant to receive all sorts of protection from the administrative heads, are killed without restriction. Thus the administrative heads under whose noses such things happen are representatives of God in name only. Such powerful administrators are rulers of the poor citizens by dress or office, but factually they are worthless, lower-class men without the cultural assets of the twice-born. No one can expect justice or equality of treatment from once-born (spiritually uncultured) lower-class men. Therefore in the age of Kali everyone is unhappy due to the maladministration of the state. The modern human society is not twice-born by spiritual culture. Therefore the people's government, by the people who are not twice-born, must be a government of Kali in which everyone is unhappy.
(Srimad Bhagavatam-----1:17:5-----purport).