2 Your letter propounds two questions, the first, why God hardened Pharaoh's heart, and why the apostle said: “So then it is not of him that wills, nor of him that runs, but of God that shows mercy;” and other things which appear to do away with free will: the second, how those are holy who are born of believing, that is, of baptized parents, seeing that without the gift of grace afterwards received and kept they cannot be saved.
Source: Letters (New Advent)