Bhakti-yoga means connecting ourselves with Krsna, God, and becoming His eternal associates. Bhakti-yoga cannot be applied to any other objective; therefore in Buddhism, for instance, there is no bhakti-yoga, because they do not recognize the Supreme Lord as the supreme objective. Christians, however, practice bhakti-yoga when they worship Jesus Christ, because they are accepting Him as the son of God and are therefor accepting God. Unless one accepts God, there is no question of bhakti-yoga. Christianity, therefor, is also a form of Vaisnavism, because God is recognized.

-His Divine Grace A.C. Bhaktivedanta Swami Prabhupada
from - The Path of Perfection