CC Madhya 20.352
kalite avatāra taiche śāstra-vākye māni
prabhu kahe — Lord Śrī Caitanya Mahāprabhu said; anya-avatāra — the incarnations in other yugas; śāstra-dvāre jāni — one has to accept by reference to the śāstras; kalite — in this Age of Kali; avatāra — incarnation; taiche — similarly; śāstra-vākye māni — one has to accept according to the description of revealed scriptures.
Śrī Caitanya Mahāprabhu replied, “As in other ages an incarnation is accepted according to the directions of the śāstras, in this Age of Kali an incarnation of God should be accepted in that way.
According to Śrī Caitanya Mahāprabhu, this is the way an incarnation should be accepted. Śrīla Narottama dāsa Ṭhākura says, sādhu-śāstra-guru-vākya, cittete kariyā aikya. One should accept a thing as genuine by studying the words of saintly people, the spiritual master and the śāstra. The actual center is the śāstra, the revealed scripture. If a spiritual master does not speak according to the revealed scripture, he is not to be accepted. Similarly, if a saintly person does not speak according to the śāstra, he is not a saintly person. The śāstra is the center for all. Unfortunately, at the present moment, people do not refer to the śāstras; therefore they accept rascals as incarnations, and consequently they have made incarnations into a very cheap thing. Intelligent people who follow Śrī Caitanya Mahāprabhu’s instructions and the instructions of the ācārya, the bona fide spiritual master, will not accept a pretender as an incarnation of God. In Kali-yuga, the only incarnation is Śrī Caitanya Mahāprabhu. Imitation incarnations take advantage of Śrī Caitanya Mahāprabhu. The Lord appeared within the past five hundred years, played as the son of a brāhmaṇa from Nadia and introduced the saṅkīrtana movement. Imitating Śrī Caitanya Mahāprabhu and ignoring the śāstra, rascals present themselves as incarnations and introduce their rascaldom as a religious process. As we have repeatedly said, religion can be given only by the Supreme Personality of Godhead. From the discussions in the Caitanya-caritāmṛta, we can understand that in different ages the Supreme Lord introduces different systems and different religious duties. In this Age of Kali, the only incarnation of Kṛṣṇa is Śrī Caitanya Mahāprabhu, and He introduced the religious duty of Kali-yuga, the chanting of the Hare Kṛṣṇa mahā-mantra: Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare.