Excerpt from His Divine Grace A.C. Bhaktivedanta Swami Prabhupada’s Lecture 1969, February 21, Los Angeles:

“Unless you accept God there is no question of bhakti-yoga. So Christian religion is also Vaiṣṇavism, because they accept God. Maybe in the, some stage, different from this. There are different stages of God realization also. The Christian religion says “God is great.” Accept! That is very good. But just how great God is, that you can understand from Bhagavad-gītā and Śrīmad-Bhāgavatam. But there is acceptance that God is great. That is, therefore, that is beginning of bhakti. You can apply that bhakti. Even the Mohammedan religion. That is also bhakti-yoga. Any religion where God is the target – that is, that can be applied in bhakti. But when there is no God, or impersonalism, there is no question of bhakti-yoga. Bhakti-yoga means bhaja dhatu kti, bhaja-sevayā. Service. Service means three things: the servitor, the served, and service. One must be present who will accept service. And one must be present to render service. And then the via media, the process of service. So bhakti-yoga means service. If there is nobody to accept the service, then where is the bhakti-yoga? So any philosophy or religious principle where there is no acceptance of God, the Supreme, there is no application of bhakti.”