Christians and Muslims are also Vaisnavas, devotees, because they offer prayers to the Lord. “O God”, they say, “give us our daily bread “. Those who offer this prayer may not know very much and may be at a lower stage, but this is a beginning, because they have approached God. Going to a church or mosque is also pious (catur-vidha bhajante mam janah sukrtino rjuna). Therefore those who begin in this way will one day become pure Vaisnavas. But the atheistic propaganda that one should not go to a church, temple, or mosque is very dangerous to human society. One may not be very advanced, but one should try at least to do something to understand God. A child is sent to school, and although he may simply learn ABCD, if he is interested he may one day become a very good scholar. Similarly, one day a pious man may become a pure devotee.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Teachings of Queen Kunti”, Page 146