What is Justification? Justification is the “gracious action of God which frees us from sin and ‘communicates the righteousness of God through faith in Jesus Christ’ and through baptism.” It is “not only the remission of sins, but also the sanctification and renewal of the interior man.” It is the “acceptance of God’s righteousness through faith in Jesus Christ,” who merited justification for us by his Passion (CCC, 1987-1992)
Romans 4:25 (RSV-CE) says that our Lord “was put to death for our trespasses and raised for our righteousness,” and Romans 5:15-16 adds, “But the free gift is not like [Adam’s] trespass. For if many died through one man’s trespass, much more have the grace of God, and the free gift in the grace of that one man, Jesus Christ, abounded for many. And the free gift is not like the effect of [Adam’s] sin. For the judgment following one trespass brought condemnation, but the free gift following many trespasses brings justification.”
Finally, Romans 5:19 says, “For as by one man’s disobedience many were made sinners, so by one man’s obedience many will be made righteous.” Now, let’s unpack a few points.
First, grace “is favor, the free and undeserved help that God gives us to respond to His call to become children of God, adoptive sons, partakers of the divine nature and of eternal life” (CCC, 1996). It is the unmerited gift of supernatural life and love. “The divine initiative in the work of grace precedes, prepares, and elicits the free response of man” (CCC, 2022) It is the righteousness of God infused into our souls. Thus, when our souls are infused with grace, we are justified.
Second, we cannot do one thing to merit this gift. We cannot work our way to grace. Even Adam and Eve were created with grace, and they certainly did nothing to merit it. In fact, if grace depended on our works, it could no longer be grace (Rom 11:6) because grace is a gift from God. When we speak of the love of God, grace is the way in which He communicates His love to us. By grace we were made, and by grace we are remade.
St. Paul writes, “…the free gift in the grace of that one man, Jesus Christ, abounded for many.” St. Thomas Aquinas illustrated this point when he wrote, “That through which one makes something is also that through which one repairs it. In practice, if a house has fallen in, one repairs it on the model through which it was originally made.” Now, Jesus is that model, and grace is the way in which he repairs our fallen nature.
Third, Romans 5:19 teaches that God doesn’t simply “cover” us with his righteousness. He makes us truly righteous. Let’s re-read Romans 5:19. Notice the connection between the actual state of sin through one man’s disobedience and the actual state of righteousness through one man’s obedience. These are actual states of the human being, not simply a covering.
 Giles Emery, O.P. The Trinitarian Theology of St. Thomas Aquinas. Oxford: Oxford University Press, 2007, 199.