"If in coming face to face with God we accept Him in our lives, then we are converting. We become a better Hindu, a better Muslim, a better Catholic, a better whatever we are. ... What God is in your mind you must accept" (from Mother Teresa: Her People and Her Work , by Desmond Doig, p. 156, as quoted by Dave Hunt, Global Peace and the Rise of Antichrist , p. 149).
What God is in your mind you must accept. So don't explain the true God of scripture, just allow them and in fact encourage them to stay with whatever they believe.