16 "So then it is not of him who wills, nor of him who runs, but of God who shows mercy. 17 For the Scripture says to the Pharaoh, “For this very purpose I have raised you up, that I may show My power in you, and that My name may be declared in all the earth.” 18 Therefore He has mercy on whom He wills, and whom He wills He hardens.
19 You will say to me then, “Why does He still find fault? For who has resisted His will?” 20 But indeed, O man, who are you to reply against God? Will the thing formed say to him who formed it, “Why have you made me like this?” 21 Does not the potter have power over the clay, from the same lump to make one vessel for honor and another for dishonor?"
Start in verse 16 and remember the context is that the Jews with the birthright are going to lose it and the wild Gentiles are going to get it. This makes no logical sense. How could the son who left home and wasted his inheritance living the fool end up home as if nothing happened? Verse 16 is saying God has the right to have mercy on anyone He wants. A legalistic person can't even comprehend mercy. They operate strictly on a legal level where everything is acquired by purchase.
Paul is trying to explain that God does not operate on that level. He offers only mercy. Those who accept His mercy begin to see the world through His eyes. Those who reject His mercy get more and more hard, like Pharaoh did. We'll talk more about his case and what we can learn from it tomorrow...