Actually, it doesn’t say in his autobiography (the Confessio) that St. Patrick didn’t practice the faith, though that’s possible. It says at least that he did a really lousy job of practicing the faith, and that he did have some really big sin going.
His specific comment about himself and other persons being captured and enslaved by the Irish was: “…we had gone away from God, and did not keep his commandments. We would not listen to our priests, who advised us about our salvation.”
So basically, he’s saying that he was a baaaaad Catholic, not that he was no Catholic. It doesn’t say that he didn’t go to church, but it does say he didn’t listen to the priests.
His other comment was about his age (16) and that “At that time, I did not know the true God.” The way he put it (“Deum veram ignorabam”) can also mean, “I ignored the true God.” Later on in the Confessio, he says about the time when he was fifteen and before that, “I did not then believe in the living God, not even when I was a child. In fact, I remained in death and unbelief until I was reproved strongly… [by being captured].”
During his slavery among foreigners, “the Lord opened up my sense of unbelief (“incredulitatis”),” and so he repented and turned to God. He was a revert, not a convert. At that point, he finally realized that God had always been there for him, even when he was a tiny kid.