Srimad Bhagavatam

References to text SB 1.3.28
SB 1.3.28: All of the above-mentioned incarnations are either plenary portions or portions of the plenary portions of the Lord, but Lord Śrī Kṛṣṇa is the original Personality of Godhead. All of them appear on planets whenever there is a disturbance created by the atheists. The Lord incarnates to protect the theists.

Text â„–
Chapter 5: How to Approach God
Teachings of Lord Caitanya
TLC 5
Chapter 23: Why Study the Vedanta-sutra?
Teachings of Lord Caitanya
TLC 23