to declare someone to be in the right: to hand down a favorable verdict, acquitting, clearing, or vindicating a person rather than making the person good
This is a courtroom word. To justify someone is to announce a verdict about them, not to change what they are inside. A judge who justifies the accused does not make that person good; the judge declares that the person stands in the right. That is the settled core of the word across the major reference works, even though its form looks as if it should mean "to make righteous." The Greek Old Testament fixed the sense: the word stands as the exact opposite of "to condemn," and since no judge condemns a person by making them wicked, its partner cannot mean making a person good either. It renders a ruling.
In the letters of Paul this verdict becomes the center of the good news. God declares guilty people to be in the right, as a free gift, on the basis of trust in Christ rather than on a record of law-keeping. The sharpest line is that God "justifies the ungodly" (Romans 4:5): the ruling is a gift, not a wage earned. Elsewhere the same verb bends to nearby uses.
- A person or a wise course of action is "vindicated," shown by results to have been right all along.
- People can "justify" God by acknowledging that he is in the right.
- A person can try to "justify himself," presenting himself as righteous to others.
The famous theological arguments that surround this word are mostly not arguments about the word itself. Whether the verdict also remakes the person from within, whose righteousness is credited to whom, and how a person receives it are debates about the salvation the word describes, not about the plain sense of the verb, which stays close to a judge clearing the accused.