Shrimad Bhagavad Gita As It Is -Shri Shrimad A.C Bhaktivedanta Swami Prabhupada
Chapter 4: Verse-8
In the Caitanya-caritämrta of Krsnadäsa Kaviräja, the following verses  summarize these principles of incarnation:
srsti-hetu yei mürti prapance avatare
“The avatära, or incarnation of Godhead, descends from the kingdom of God for material manifestation. And the particular form of the Personality of Godhead who so descends is called an incarnation, or avatära. Such incarnations are situated in the spiritual world, the kingdom of God. When they descend to the material creation, they assume the name avatära.” There are various kinds of avatäras, such as purusävatäras, gunävatäras, lilävatäras, sakty-ävesa avatäras, manvantara-avatäras and yugävatäras—all appearing on schedule all over the universe. But Lord krsna is the primeval Lord, the fountainhead of all avatäras. Lord Krsna descends for the specific purpose of mitigating the anxieties of the pure devotees, who are very anxious to see Him in His original Vrndävana pastimes. Therefore, the prime purpose of the krsna avatära is to satisfy His unalloyed devotees.
The Lord says that He incarnates Himself in every millennium. This indicates that He incarnates also in the Age of Kali. As stated in the Srimad-Bhägavatam, the incarnation in the Age of Kali is Lord Caitanya Mahäprabhu, who spread the worship of krsna by the sankirtana movement (congregational chanting of the holy names) and spread krsna consciousness throughout India. He predicted that this culture of sankirtana would be broadcast all over the world, from town to town and village to village. Lord Caitanya as the incarnation of Krsna, the Personality of Godhead, is described secretly but not directly in the confidential parts of the revealed scriptures, such as the Upanisads, Mahäbhärata and Bhägavatam. The devotees of Lord Krsna are very much attracted by the sankirtana movement of Lord Caitanya. This avatära of the Lord does not kill the miscreants, but delivers them by His causeless mercy.
- Madhya 20.263–264