SB 8.5.46
tais taih sveccha-bhutai rupaih
kale kale svayam vibho
karma durvisaham yan no
bhagavams tat karoti hi
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O Lord, O Supreme Personality of Godhead, by Your sweet will You appear in various incarnations, millennium after millennium, and act wonderfully, performing uncommon activities that would be impossible for us.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Lord says in Bhagavad-gita (4.7):
yada yada hi dharmasya
glanir bhavati bharata
abhyutthanam adharmasya
tadatmanam srjamy 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.” Thus it is not an imagination but a fact that the Supreme Personality of Godhead, by His sweet will, appears in different incarnations, such as Matsya, Kurma, Varaha, Nrsimha, Vamana, Parasurama, Ramacandra, Balarama, Buddha and many other forms. Devotees are always eager to see one of the Lord’s innumerable forms. It is said that just as no one can count how many waves there are in the sea, no one can count the forms of the Lord. This does not mean, however, that anyone can claim to be a form of the Lord and be acceptable as an incarnation. The incarnation of the Supreme Personality of Godhead must be accepted in terms of the descriptions found in the sastras. Lord Brahma is eager to see the incarnation of the Lord, or the original source of all incarnations; he is not eager to see an imposter. The incarnation’s activities are proof of His identity. All the incarnations described in the sastras act wonderfully (kesava dhrta-mina-sarira jaya jagadisa hare). It is only by the personal sweet will of the Supreme Personality of Godhead that He appears and disappears, and only fortunate devotees can expect to see Him face to face.