It’s a reference to Paul, from the Bible (https://en.wikipedia.org/wiki/Paul_the_Apostle#Names). He was known as Saul when he was Jewish, and then later went by Paul after he converted to Christianity. The phrase “Saul to Paul” is often used to refer to someone who converted to Christianity, as opposed to being born a Christian. But it is also sometimes used more generally to mean something that turned around or improved. Like, if you want to say “glow up” but you also want to make it clear that you think Christians are better than Jews.

Ironically, the person who said it was Mormon, and most Christians do not consider Mormons to count as Christian.