God appears for four main reasons:

    1. To associate with His devotees. He wants to enjoy by playing and living among His creations. He has already appeared more than 18,000 in His original form and millions of times in His other forms in the current cycle. Each cycle is of 311.040 trillion years duration and so far we have passed 155.522 trillion years in the current cycle. He has also appeared millions of times in each of the previous cycles (zillions of cycles gone). 
    2. To defeat the agnostics. “Agnostic” in this sense means those who believe God is unknown or unknowable. There are many foolish people who conclude that God is unknown, simply because they themselves don’t know. Everything is not limited to our limited perception and abilities. God can be seen and known by those who care to know.
    3. To defeat the atheists. “Atheists” are those who believe there is no God.
    4. To destroy the demonic. Every time Lord Krishna appears, He saves His pure devotees and destroys the demonic. He does not come to give post dated checks. He pays on the spot.

 

“Although I am unborn and My transcendental body never deteriorates, and although I am the Lord of all living entities, I still appear in every millennium in My original transcendental form.” (Lord Krishna, Bhagavad-Gita 4.6)

Millennium in the above verse refers to every 8.64 billion years. Lord Krishna appears in His original form (Godhead), once very 8.64 billion years.

“To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium.” (Lord Krishna, Bhagavad-Gita 4.8)

Millennium in the above verse refers to every Yuga. There are four Yugas; Satya, Tetra, Dvarpa, Kali. We are currently 5,000 years into Kali Yuga.

“O son of Kuntī, at the end of the millennium all material manifestations enter into My nature, and at the beginning of another millennium, by My potency, I create them again.” (Lord Krishna, Bhagavad-Gita 9.7)

Millennium in the above verse refers to end or start of universe. At the beginning of each cycle, the universes come out from the body pores of Lord Krishna in His Maha Vishnu form. At the end of the cycle, the universes go back into His body. Each cycle is 311.040 trillion years duration.

“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. To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium.” (Lord Krishna, Bhagavad-Gita 4.7-8)

“Fools deride Me when I descend in the human form. They do not know My transcendental nature as the Supreme Lord of all that be.” (Lord Krishna, Bhagavad-Gita 9.11)

The devotees of Lord Krishna also have to defeat the agnostics and atheists by spreading the teachings of Lord Krishna as in the Bhagavad-Gita.