Augustine of Hippo

Now this, I think, he writes to the Galatians, to whom he says, "Who gave you the Spirit." … From this it is apparent that he is speaking to Christians, people to whom God had given the Spirit, and therefore to the baptized. See, the sinful nature is an adversary even within the baptized, and there is not in them that possibility [of sinlessness] which [Pelagius25] says is so implanted in our nature that it cannot be annulled.