For he says to Moses,
I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion.
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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. Commentary on Paul’s Epistles.
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.
I will have mercy Then it is not of him that willeth By these words he again teaches that God's call and predestination of those whom he has decreed to save, is not upon account of any works or merits in men, but only to be attributed to the mercy and goodness of God. See St. Thomas Aquinas on this chap. lect. iii. See St. Augustine, Encher. chap. xcviii. Epis. 194. in the new Ed. Ep. 105. ad Sixtum de lib. Arbit. chap. xxv. (Witham)
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! Homilies on Romans
