Question: If you want to pray that someone who is not Catholic will convert to Catholicism, which saint would be the one ask for their intercession?

Thank you for this great question. First of all, allow me to commend you for your loving concern for your friend. Let me also commend you for realizing the importance of prayer in this case. None of us can convert anyone. Only God, through the grace of Jesus Christ moves hearts. So in this case, as in all cases, prayer is the most important thing we can do and it must be the foundation of anything else that we do.

The overall answer is that any Saint’s intercession would be powerful in this case. But my personal suggestion (and others may have differing opinions on this), is that you should invoke the special intercession of St. Augustine. He was a Saint who lived a very immoral life before his conversion, but afterward became a great defended of the Church. His intercession would be very powerful in this case. You can read more about his life at: http://www.catholic.org/saints/saint.php?saint_id=418.