9:15 saith to Moses. This was said to Moses in response to Moses’ request to see the glory of God (Exodus 33:18-19). Since there are none who merit salvation, there are none who are fit (or who can make themselves fit) to be in God’s immediate presence. Therefore, it is entirely by God’s mercy that any at all are saved, and He surely has the divine right by creation to show mercy on whomever He will (Romans 9:16).

9:17 scripture saith unto Pharaoh. Note here that Paul equates the word spoken by God (Exodus 9:16) with “Scripture.” The fact that this pronouncement of God was written down by Moses (the Greek word for “Scripture” was graphe, meaning a “written document”) gave the writing the authority of God Himself. Note Paul’s summary in II Timothy 3:16,17.