In the Satya-yuga the process of self-realization was meditation, and this process was taught by the white incarnation of God. This incarnation gave a benediction to the sage Kardama by which he obtained an incarnation of the Personality of Godhead as his son. In the Satya-yuga everyone meditated on Kṛṣṇa, and each and every living entity was in full knowledge. In the present age, the Kali-yuga, people who are not in full knowledge are still attempting various meditative processes not recommended for this age.
In the Tretā millennium the process for self-realization was the performance of various sacrifices, and this process was taught by the red incarnation of God.
In the Dvāpara millennium Kṛṣṇa was personally present, and the process of self-realization for everyone in that age was worshiping Him. He was blackish in color, He was the incarnation of the Supreme Personality of Godhead Himself, and He induced people to worship Him, as stated in the Bhagavad-gītā. Śrīmad-Bhāgavatam (11.5.29) states that in the Dvāpara millennium people generally worshiped the Supreme Lord Kṛṣṇa by chanting the following hymn:
- namas te vāsudevāya namaḥ saṅkarṣaṇāya
- pradyumnāyāniruddhāya tubhyaṁ bhagavate namaḥ
"Let me offer my obeisances unto the Supreme Personality of Godhead in the forms of Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha."
In the next millennium—the present age, Kali-yuga—the Lord is yellow, and He teaches people the process of attaining love of God by chanting the names of Kṛṣṇa. This process is shown personally by Kṛṣṇa in the form of Caitanya Mahāprabhu, and He exhibits love of Godhead by chanting, singing and dancing with thousands of people following Him. This particular incarnation of the Supreme Personality of Godhead is foretold in Śrīmad-Bhāgavatam (11.5.32):
- kṛṣṇa-varṇaṁ tviṣākṛṣṇaṁ sāṅgopāṅgāstra-pārṣadam
- yajñaiḥ saṅkīrtana-prāyair yajanti hi su-medhasaḥ
“In the Age of Kali the Lord incarnates as a devotee and is always chanting Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. Although He is Kṛṣṇa, His complexion is not blackish like Kṛṣṇa's in Dvāpara-yuga but is golden. He always engages in preaching love of Godhead through the saṅkīrtana movement, and those who are intelligent adopt this process of self-realization.” Śrīmad-Bhāgavatam (12.3.52) also states:
- kṛte yad dhyāyato viṣṇuṁ tretāyāṁ yajato makhaiḥ
- dvāpare paricaryāyāṁ kalau tad dhari-kīrtanāt
"The self-realization achieved in the Satya millennium by meditation, in the Tretā millennium by the performance of different sacrifices, and in the Dvāpara millennium by worship of Lord Kṛṣṇa can be achieved in the Age of Kali simply by chanting the holy names, Hare Kṛṣṇa." This is confirmed in the Viṣṇu Purāṇa (6.2.17):
- dhyāyan kṛte yajan yajñais tretāyāṁ dvāpare ’rcayan
- yad āpnoti tad āpnoti kalau saṅkīrtya keśavam
"In this age there is no use in meditation, performance of sacrifices, or temple worship. Simply by chanting the holy name of Kṛṣṇa—Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare—one can achieve perfect self-realization."
When Lord Caitanya described the incarnation for this Age of Kali, Sanātana Gosvāmī, who had been a government minister and was perfectly capable of drawing conclusions, asked the Lord, "How can one understand the advent of an incarnation?" From the description of the incarnation for the Kali millennium, Sanātana Gosvāmī could understand that Lord Caitanya Himself was that incarnation of Kṛṣṇa, and he could also understand that in the future many people would try to imitate Lord Caitanya because the Lord had played as an ordinary brāhmaṇa though His devotees accepted Him as an incarnation. Since Sanātana knew that there would be many pretenders, he asked the Lord, "How can one understand the symptoms of an incarnation?"
"As one can understand the different incarnations for previous millenniums by referring to the Vedic literature," the Lord replied, "one can similarly understand who is actually the incarnation of Godhead in this Age of Kali." In this way the Lord especially stressed reference to authoritative scriptures. In other words, one should not whimsically accept a person as an incarnation but should try to understand the characteristics of an incarnation by referring to the scriptures. An incarnation of the Supreme Lord never declares Himself to be an incarnation, but His followers must ascertain who is an incarnation and who is a pretender by referring to authoritative scriptures.
Any intelligent person can understand the characteristics of a real incarnation by understanding two kinds of features—the principal features, called personal characteristics, and the marginal features, comprising His activities. The scriptures describe both kinds of features. For example, in the beginning of Śrīmad-Bhāgavatam (1.1.1), the features of an incarnation are nicely described. In that verse, the two terms param (supreme) and satyam (truth) are used, and Lord Caitanya indicated that these words reveal Kṛṣṇa's principal feature. The other, marginal features are that He taught Vedic knowledge to Brahmā and incarnated as the puruṣa-avatāra to create the cosmic manifestation. These are occasional features manifested for some special purposes. One should be able to understand and distinguish the principal and marginal features of an avatāra. No one can declare himself an incarnation without referring to these two features. An intelligent man will not accept anyone as an avatāra without studying the principal and marginal features. When Sanātana Gosvāmī tried to confirm Lord Caitanya's personal characteristics as being those of the incarnation for this age, Lord Caitanya Himself indirectly confirmed Sanātana's conclusion by simply saying, "Let us leave aside all these discussions and continue with a description of the śaktyāveśa-avatāras."