It is not unjust for God to have mercy on those he wishes to have mercy on but not on others. For, as Paul says, God demonstrates through Moses what his mercy was like. He does not dispense mercy according to human standards, but according to the wisdom of God. For we are shown mercy not because of our own works but because of God, who has the power to show mercy.
This means that God will have mercy on those whom he knows will be converted and remain with him.… He will show mercy to those who, after they have sinned, return to him with a right heart. It is God’s to give or to not give. He calls the ones whom he knows will obey and does not call those whom he knows will not obey.
Here again he adds force to the objection by dividing it in two and meeting it, and starting another fresh difficulty. But to make what I have said clearer, one must needs explain it. God, he means, said that "the elder shall serve the younger," before the travail. What then? "Is God unrighteous?" By no means. Now listen to what follows also. For in that case the virtue or the vice, might be the decisive thing. But here there was one sin on which all the Jews joined, that of the molten calf, and still some were punished, and some were not punished. And this is why He says, "I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion." [Exodus 33:19: observe context] For it is not yours to know, O Moses, he means, which are deserving of My love toward man, but leave this to Me. But if Moses had no right to know, much less have we. And this is why he did not barely quote the passage, but also called to our minds to whom it was said. For it is Moses, he means, that he is speaking to, that at least by the dignity of the person he might make the objector modest. Having then given a solution of the difficulties raised, he divides it in two, by bringing forward another objection besides, as follows:
God was telling Moses that it was not his to know who was deserving of God’s love towards man; rather, Moses was to leave that up to God. If that was true for him, how much more is it true for us!
God was merciful to us in the first place in that he called us while we were still sinners … and he continues to have mercy on us now that we believe. How does God have mercy a second time? He gives his Holy Spirit to the man who believes and asks for him. And having given the Spirit God will then have compassion on those to whom he has already shown compassion. That is to say, he will make the believer compassionate so that he may do good works through love. Let no one take the credit for acting compassionately, since it was by the Holy Spirit that God gave him this love, without which no one can be compassionate.God did not elect those who had done good works, but those who believed, so that he might enable them to do good works. It is our part to believe and to will and his part to give to those who believe and will the ability to do good works through the Holy Spirit, by whom the love of God is poured out in our hearts in order to make us compassionate.
[AD 215] Clement of Alexandria on Romans 9:15