I never heard Mother Theresa say anything like this. Got evidence to support your claim?
In the biography Mother Teresa: Her People and Her Work, she is quoted by Desmond Doig as follows: “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 approach would I use? For me, naturally, it would be a Catholic one, for you it may be Hindu, for someone else, Buddhist, according to ones conscience. What God is in your mind you must accept” (Doig, Mother Teresa, Harper & Row, 1976, p. 156).
My secondary source, which contains a full article regarding her theology, is wayoflife.org.