When St. Paul talks about God’s preparing vessels of wrath and vessels of mercy as a potter who has power over clay, he seems to make God out as almost arbitrary. It seems terribly harsh, as though God says to Himself, “This one will do, I’ll show mercy to her; but the rest are out of luck.” If all you have is Romans chapter nine, it looks pretty grim. However, writing to Jewish Christians at Rome, St. Paul knew that his readers were familiar with the Prophet Jeremiah's comparison of God to a potter, and it is this knowledge that helps us understand why God makes some “vessels of wrath prepared for destruction” and others “vessels of mercy.”
In Jeremiah 18 God sends Jeremiah to watch a potter throw clay. First the potter tries to make one kind of vessel, but the clay does not cooperate: it won’t shape the way the potter is pushing it. So the potter starts again and shapes something else with the same lump of clay. Then God tells Jeremiah to tell the people, “Behold, you are as clay to the potter in my hands: I shall declare a decree...to destroy...but if [they] turn away from all their evils, then I will repent concerning the calamities I considered to do to them. If I shall declare a decree … to rebuild and plant… but [they] do evils in My sight so as not to hear My voice, then I will repent concerning the good I spoke to do to them.”
Notice that twice in this passage, God says that He will declare one thing and repent based on the response of the people. God repent? Yes, that’s what He says.
You can see this divine repentance at work in the book of Jonah. Jonah, eventually, after the terrible ordeal in the belly of a sea monster, preaches to the Ninevites: “Three days and the Lord will destroy this city!” What do the people do? They humble themselves by fasting, sackcloth (the very uncomfortable clothing of the poorest people) and ashes (not bathing). They cry out to God for mercy. And God shows mercy, or to use the words of St. Paul, God makes them vessels of mercy. At the time of Jeremiah, however, Israel with all of God’s promises to abide as the apple of His eye forever, turn to evil. So what does God do? He brings destruction, He makes them vessels of wrath. But even at the last minute, Jeremiah is told to tell the people that repentance is possible. Even at the eleventh hour, they could humble themselves and return to God and be saved.
When St. Paul says that God endures with great patience vessels of wrath, he is referring to God’s patience in allowing people multiple opportunities to repent--so that God might repent. However, “God will not always strive with man,” and eventually God lets us all have what we want.