The devotees are constantly engaged in the Supreme Lord's service. The Lord understands the mentality and sincerity of a particular living entity who is engaged in Kṛṣṇa consciousness and gives him the intelligence to understand the science of Kṛṣṇa in the association of devotees. Discussion of Kṛṣṇa is very potent, and if a fortunate person has such association and tries to assimilate the knowledge, then he will surely make advancement toward spiritual realization. Lord Kṛṣṇa, in order to encourage Arjuna to higher and higher elevation in His potent service, describes in this Ninth Chapter matters more confidential than any He has already disclosed.
(Bhagavad-Gita----9:1----purport).

Antaḥ-praviṣṭaḥ śāstā janānām. The living entity forgets as soon as he quits his present body, but he begins his work again, initiated by the Supreme Lord. Although he forgets, the Lord gives him the intelligence to renew his work where he ended his last life. So not only does a living entity enjoy or suffer in this world according to the dictation from the Supreme Lord situated locally in the heart, but he receives the opportunity to understand the Vedas from Him. If one is serious about understanding the Vedic knowledge, then Kṛṣṇa gives the required intelligence. Why does He present the Vedic knowledge for understanding? Because a living entity individually needs to understand Kṛṣṇa. Vedic literature confirms this: yo 'sau sarvair vedair gīyate. In all Vedic literature, beginning from the four Vedas, Vedānta-sūtra and the Upaniṣads and Purāṇas, the glories of the Supreme Lord are celebrated. By performance of Vedic rituals, discussion of the Vedic philosophy and worship of the Lord in devotional service, He is attained. Therefore the purpose of the Vedas is to understand Kṛṣṇa.
(Bhagavad-Gita----15:15----purport).

As stated in Bhagavad-gītā (10.10), the Lord gives intelligence to the pure devotees so that they may be elevated to the highest perfectional stage. It is confirmed herein that a pure devotee, who constantly engages in the loving service of the Lord, is awarded all knowledge necessary to reach the Supreme Personality of Godhead. For such a devotee there is nothing valuable to be achieved but the Lord's service. If one serves faithfully, there is no possibility of frustration because the Lord Himself takes charge of the devotee's advancement. The Lord is seated in everyone's heart, and He knows the devotee's motive and arranges everything achievable. In other words, the pseudo devotee, who is anxious to achieve material gains, cannot attain the highest perfectional stage because the Lord is in knowledge of his motive. One merely has to become sincere in his purpose, and then the Lord is there to help in every way.
(Srimad Bhagavatam----3:13:49----purport).

As stated in Bhagavad-gītā, intelligence and forgetfulness are both supplied by the Supersoul sitting with the individual soul within the body. When He sees that a conditioned soul is very serious about getting out of the clutches of the material influence, the Supreme Lord gives intelligence internally as Supersoul and externally as the spiritual master, or, as an incarnation of the Personality of Godhead Himself, He helps by speaking instructions such as Bhagavad-gītā. The Lord is always seeking the opportunity to reclaim the fallen souls to His abode, the kingdom of God. We should always feel very much obliged to the Personality of Godhead, for He is always anxious to bring us into the happy condition of eternal life. There is no sufficient means to repay the Personality of Godhead for His act of benediction; therefore, we can simply feel gratitude and pray to the Lord with folded hands.
(Srimad Bhagavatam----3:31:18----purport).

The Lord is so merciful that not only does He fulfill the desires of a devotee who is driven by ignorance and desires for material benefit, but He also gives such a devotee all protection, just as a cow gives milk to a newly born calf. In the Bhagavad-gītā it is said that the Lord gives intelligence to the constantly engaged devotee so that he may gradually approach the Lord without difficulty. A devotee must be very sincere in his devotional service; then, although there may be many things wrong on the devotee's part, Kṛṣṇa will guide him and gradually elevate him to the highest position of devotional service.
(Srimad Bhagavatam----4:9:17----purport).

A devotee completely surrenders unto the Supreme, who reveals Himself by His causeless mercy. As stated in Bhagavad-gītā, dadāmi buddhi-yogaṁ tam. The Lord says, "I give him intelligence." What is that intelligence? Yena mām upayānti te. The Lord gives one the intelligence to cross over the ocean of nescience and come back home, back to Godhead. In conclusion, the cause of all causes, the Absolute Truth, or Supreme Brahman, cannot be understood by philosophical speculation, but He reveals Himself to His devotee because the devotee fully surrenders unto His lotus feet. Bhagavad-gītā is therefore to be accepted as a revealed scripture spoken by the Absolute Truth Himself when He descended to this planet. If any intelligent man wants to know what God is, he should study this transcendental literature under the guidance of a bona fide spiritual master. Then it is very easy to understand Kṛṣṇa as He is.
(Srimad Bhagavatam----4:11:23----purport).

"I am seated in everyone's heart, and from Me come remembrance, knowledge and forgetfulness." The Lord is present in everyone's heart, and He gives the living entity intelligence. According to the desires of the living entity, the Lord makes him remember or forget. If the living entity is demoniac and wants to forget the Supreme Personality of Godhead, the Lord gives him the intelligence to be able to forget the Supreme Lord forever. Similarly, when a devotee wants to serve the Supreme Lord, the Lord, as Paramātmā, gives the devotee the intelligence to make progress in devotional service. The Lord directly witnesses our activities and experiences our desires. The Supreme Lord gives us the facilities to act in the way we wish.
(Srimad Bhagavatam----4:30:26----purport).

In Bhagavad-gītā (15.15) it is said, mattaḥ smṛtir jñānam apohanaṁ ca: the Supreme Personality of Godhead in His localized aspect gives intelligence to the individual soul as far as he is able to grasp it. Therefore we find the individual soul in different high and low positions. A living entity with the body of a bird or beast cannot take instructions from the Supreme Soul as adequately as an advanced human being. Thus there are gradations of bodily forms. In human society, the perfect brāhmaṇa is supposed to be the most advanced in spiritual consciousness, and further advanced than the brāhmaṇa is the Vaiṣṇava.
(Srimad Bhagavatam----7:14:38----purport).

Attachment, greed and lust are three formidable forces that prevent one from concentrating upon the lotus feet of the Supreme Personality of Godhead. These forces act because the Supreme Lord does not like to be realized by nondevotees and atheists. However, when one surrenders unto the lotus feet of the Lord, these impediments are withdrawn, and one can realize the Supreme Personality of Godhead. Therefore the Lord is the protector of the surrendered soul. One cannot become a devotee until one surrenders unto the Lord's lotus feet. Then the Lord gives one the intelligence from within by which one can return home, back to Godhead.
(Srimad Bhagavatam----8:3:28----purport).

The Lord Himself claims in the Gītā that He is the seed-giving father of all forms and species, who therefore must all be considered sons of the Lord. The entrance of the Supreme Lord into everyone's heart as Paramātmā sometimes bewilders the impersonalists into equating the living entities with the Supreme Lord. They think, "Both the Supreme Lord and the individual soul enter into the various bodies; so where is the distinction? Why should individual souls worship the Paramātmā, or Supersoul?" According to them, the Supersoul and the individual soul are on the same level; they are one, without any difference between them. There is a difference, however, between the Supersoul and the individual soul, and this is explained in the Bhagavad-gītā, Fifteenth Chapter, wherein the Lord says that although He is situated with the living entity in the same body, He is superior. He is dictating or giving intelligence to the individual soul from within. It is clearly stated in the Gītā that the Lord gives intelligence to the individual soul and that both memory and forgetfulness are due to the influence of the Supersoul. No one can act independently of the sanction of the Supersoul. The individual soul acts according to his past karma, reminded by the Lord. The nature of the individual soul is forgetfulness, but the presence of the Lord within the heart reminds him of what he wanted to do in his past life. The intelligence of the individual soul is exhibited like fire in wood. Although fire is always fire, it is exhibited in a size proportionate to the size of the wood. Similarly, although the individual soul is qualitatively one with the Supreme Lord, he exhibits himself according to the limitations of his present body. But the Supreme Lord, or the Supersoul, is unlimited.
(Krsna Book).