What then shall we say? Is God unjust? Not at all! For he says to Moses,
“I will have mercy on whom I have mercy,
and I will have compassion on whom I have compassion.”
It does not, therefore, depend on human desire or effort, but on God’s mercy. For Scripture says to Pharaoh: “I raised you up for this very purpose, that I might display my power in you and that my name might be proclaimed in all the earth.” 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 is able to resist his will?” But who are you, a human being, to talk back to God? “Shall what is formed say to the one who formed it, ‘Why did you make me like this?’” Does not the potter have the right to make out of the same lump of clay some pottery for special purposes and some for common use?
Romans 9:14-21 (NIV)