Kṛṣṇa says that dharma-saṁsthāpanārthāya sambhavāmi yuge yuge. Now, one may inquire that, "Kṛṣṇa appeared in the Tretā-yuga. And when He is going to appear in the Kali-yuga?" That is also mentioned in the Śrīmad-Bhāgavatam, that in the Kali-yuga there are three different incarnations mentioned in the Bhāgavatam or any other authentic Vedic literature. One incarnation is Lord Buddha, and another incarnation is Lord Caitanya, and another incarnation, in the last stage of this age, is Kalki, so far we get from the authority of Bhāgavatam.
So we have to accept according to the authentic scripture who is incarnation. We cannot accept anyone who claims that, "I am also incarnation." No. In the śāstras there are symptoms foretold of the incarnation. Just like about Lord Buddha there is mention, "In such and such place, in such and such form, in such and such activity, Lord Buddha will appear." Similarly, about Lord Caitanya is there. Similarly, there is a description about Kalki.
So far Lord Buddha is concerned, in the Bhāgavatam the name of the mother of Lord Buddha is mentioned there. And the activities is also mentioned, what are his activities. The activities are not very pleasant. Sammohāya sura-dviṣām (SB 1.3.24):
"The Lord will appear as Buddha in order to sammohāya, bewilder, the atheist class of men." Atheist class of . . . his activities were to cover the atheist class of men, those who do not believe in God. Yes. Lord Buddha said, "Yes, there is no God. There is no God. There is void only. But you believe me, what I say." Just see.