Therefore God has mercy on whom he wants to have mercy, and he hardens whom he wants to harden. One of you will say to me: “Then why does God still blame us? For who resists his will?” But who are you, O man, to talk back to God? “Shall what is formed say to him who formed it, ‘Why did you make me like this?’”
Salvation is God's choice. There is no doubt about that. However, what people cannot accept is its corollary: condemnation is also God's choice.
One implication is the question asked in verse 19. Paul's answer isn't very comforting, even though it is deeply rooted in Old Testament Scripture (Isa 29:16, 45:9, Job 38-42). How on Earth is God justified to do this? Is He just a tyrant?
God is God. Whatever he does, he does out according to his good pleasure. What pleases God? Certainly not wickedness! He is good, holy, and righteous. He is a God who is for us that none may stand against us.
How do we know this? Because Jesus Christ is the full revelation of God. The Scriptures are his divinely inspired self revelation because he is a God who wants to be known for who he is and who he is is perfect, pleasing, and good. Therefore, we have no need to question God's motives, nor is it our place. Who can understand why God does what he does? God is who He says He is and that is enough.