As Lord Kṛṣṇa states in the Bhagavad-gītā (4.7):

yadā yadā hi dharmasya glānir bhavati bhārata
abhyutthānam adharmasya tadātmānaṁ sṛjāmy aham

“Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion — at that time I descend Myself.”

This is also the case with Caitanya Mahāprabhu’s appearance. Śrī Caitanya Mahāprabhu appeared in this world as a disguised incarnation of Kṛṣṇa, but His appearance is confirmed in Śrīmad-Bhāgavatam, the Mahābhārata and other Vedic scriptures. He appeared in order to teach the fallen souls in this material world, for in this Age of Kali almost everyone has become attached to fruitive and ritualistic activities and mental speculation. Consequently there was a great need to revive the system of devotional service. The Lord Himself personally came down disguised as a devotee so that the fallen populace might take advantage of the Lord’s example.

At the conclusion of the Bhagavad-gītā, Lord Kṛṣṇa advised complete surrender unto Him, promising all protection to His devotee. Unfortunately, people are so fallen that they cannot accept the instructions of Lord Kṛṣṇa; therefore Kṛṣṇa returned with the same mission, but He executed it in a different way. As Lord Śrī Kṛṣṇa, the Supreme Personality of Godhead, He ordered us to surrender unto Himself, but as Lord Śrī Caitanya Mahāprabhu, He taught us how to surrender to Kṛṣṇa. Therefore He is praised by the Gosvāmīs: namo mahā-vadānyāya kṛṣṇa-prema-pradāye te. Lord Śrī Kṛṣṇa is certainly the Personality of Godhead, but He is not as magnanimous as Śrī Caitanya Mahāprabhu. Lord Kṛṣṇa simply gave orders for one to become His devotee (man-manā bhava mad-bhaktaḥ), but Śrī Caitanya Mahāprabhu actually taught the process of Kṛṣṇa consciousness. If one wants to become a devotee of Kṛṣṇa, he must first take shelter of the lotus feet of Śrī Caitanya Mahāprabhu, following in the footsteps of Sārvabhauma Bhaṭṭācārya and other exalted devotees.