John Chrysostom

What does "Let us have peace" mean? Some say, "Let us not be at variance, through a peevish obstinacy for bringing in the Law." But to me he seems to be speaking now of our conversation. For after having said much on the subject of faith, he had set it before righteousness which is by works, to prevent any one from supposing what he said was a ground for listlessness, he says, "let us have peace," that is, let us sin no more, nor go back to our former estate. For this is making war with God. …

Homily on Romans IX ↗

Theophylact of Ohrid

Here the Apostle reasons about the life of faith, so that after he has said so much in praise of faith and disparaged works, we would not become negligent. Since faith has justified us, let us sin no more, but "let us have peace with God" through a life pleasing to Him. How will this be? "Through our Lord Jesus Christ." He who justified us when we were sinners will also help us to be preserved in His righteousness.

Commentary on Romans ↗

Thomas Aquinas

After showing the need for Christ's grace, because without it neither the knowledge of the truth benefited the gentiles nor circumcision and the law benefited the Jews unto salvation, the Apostle now begins to extol the power of grace. Concerning this, he does two things. First, he shows what goods we obtain through grace; second, from what evils we are freed by it, at wherefore as by one man (Rom 5:12). In regard to the first he does two things. …

Commentary on Romans ↗

Bonaventure

The first fruit of grace is the remission of guilt. Whence the Apostle to the Romans: "Justified by faith, let us have peace with God through our Lord Jesus Christ, through whom also we have access by faith into this grace, in which we stand and glory in the hope of the glory of the sons of God." As the Philosopher holds, one is not changed from viciousness to virtues except through habituation to the contrary habit; so guilt will never be remitted except through grace. …

Collationes de Septem Donis, Collation 1 ↗