What does Romans 9:20 mean?

Paul now asks his readers, all of us, some hard questions. He has imagined that we are responding to the example of God hardening Pharaoh's heart, yet still holding Pharaoh at fault, with questions about God's fairness. This is a normal human reaction; if a person somehow "forced" another into something, we'd consider it outrageous to hold the coerced person responsible.

But Paul turns the question back on mankind: Who are we, as mortal human beings, to answer back to God? God is the One who molded Adam from the dust of the ground (Genesis 2:7) and who puts all of us together in our mother's womb (Psalm 139:13). Can the one who is molded talk back to the One who molded him and demand he ought to have been made in some other way?

The assumed answer, of course, is no. Created things don't talk back to their maker. Neither do human beings have the right to moralize to their Creator about His choices. He is God. We are not. As crippling as it might be to our own sense of pride, we must start with the realization that God has no obligation to us. He owes us nothing: not mercy, not love, not grace. That, in fact, is one reason the gospel is so incredible. The love and mercy God shows to us, in providing for our salvation, is something absolutely and completely unearned and undeserved.

We can't appreciate the depths of that kind of love until we accept the fact that it's entirely unnecessary on God's part.