And that he might make known the riches of his glory on the vessels of mercy, which he had before prepared unto glory,
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It is God’s patience and longsuffering that, just as he prepares the wicked for destruction, so also he prepares the good for their reward. For the good are those who have the hope of faith. God preserves everyone knowing what the destiny of each will be. Therefore, it is a sign of his patience that those who have been rescued from evil or who persevere in good works he prepares for glory. Commentary on Paul’s Epistles.
In giving to some what they did not deserve God obviously wanted his grace to be gratuitous and therefore genuinely grace, and in not giving it to all he showed what all deserved. He is good in the benefit given to certain people and just in the punishment of others but good in all things, for it is good when that which is deserved is given, and just in all things, as it is just when that which is not merited is given without injury to anyone.
We who were vessels of wrath through our first birth have deserved to become vessels of mercy through the second one. The first birth brought us forth unto death, but the second one recalled us to life. All of us were temples of the devil before baptism, but after baptism we were made ready to become temples of Christ.
The Jews reproached the Gentiles because the latter were saved by grace, and they thought that by making this accusation they would bring shame on them. But Paul sets this insinuation aside, because if this brought glory to God, how much more would it bring glory to those through whom God was glorified? Homilies on Romans