When you want to convert Christians into Krsna consciousness, you should understand first of all the philosophy of Krsna consciousness. Without understanding the philosophy of Krsna consciousness, if we try to convert Christians into Krsna consciousness, it will be utter failure. We do not condemn any religion because the Bhagavatam says that the religious process is the best by which one can attain love of God. So we are teaching love of Godhead, not any particular type of religion. Our Krsna consciousness movement is not a religious movement; it is a movement for purifying the heart.
From a letter by Srila Prabhupada to Yadunandana – San Francisco 13 April, 1968