The cause of the Supreme Lord’s appearance upon this earth is stated in Bhagavad-gītā (4.7-8) by the Lord Himself:

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

paritrāṇāya sādhūnāṁ vināśāya ca duṣkṛtām dharma-saṁsthāpanārthāya sambhavāmi yuge yuge

“Whenever and wherever there is a decrease in religious principles and a predominant rise in irreligion, at that time I descend Myself. To deliver the pious and annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium.” The Lord appears in order to execute two kinds of activities — to kill the demons and to protect the devotees. When the devotees are too disturbed by the demons, the Lord certainly appears in different incarnations to give the devotees protection. The devotees following in the footsteps of Prahlāda Mahārāja should not be disturbed by the demoniac activities of the nondevotees. Rather, they should stick to their principles as sincere servants of the Lord and rest assured that the demoniac activities directed against them will not be able to stop their devotional service.