In Paul’s letters, the verb “justify” (Gr. dikaioo) means “acquit, judge favorably,” with overtones of forgiveness and reconciliation. It has an invariably positive meaning.
Paul didn’t learn to use “justify” that way from ancient or contemporary Greek. As James Prothro explains in a 2016 article, outside the Septuagint and the New Testament, the verb takes both impersonal and personal objects. In the first case, the verb means “to deem a thing or action dikaios,” and the latter would mean anything from “observant of custom” to “legally entitled” to aesthetically “fitting.” The force of the verb depends on the subject who does the justifying. When a judge “deems right” a course of action, the decision has the force of a decree that ought to be followed (dikaioma). On the other hand, “when the subject has no official say in the course of events, to ‘justify’ is more a matter of ‘estimation, valuation, or choice.’” When the word group is used in situations of conflict or debate, dikaioo “entails claiming something to be right over against alternatives.”
All very interesting, but the real interest is in the verb’s usage with personal objects. The normal meaning is “‘punish’ or ‘condemn,’ i.e. to enact just retribution upon a malefactor.” Prothro finds one passage in Aristotle where the verb means “to be treated well,” but apart from that one exception “to ‘justify’ someone or to ‘be justified’ appears to be a set expression in judicial scenes before a judge or ruler,” where “it is at best neutral, referring to the exercise of judicial authority in general; it is never used of the act whereby a sovereign decides or acts in favor of a person.” It is paralleled by verbs like “put to death” and “punish,” and set in opposition to verbs like “release”: “Within this semantic field, it would be better to gloss this with the English idiom ‘bring someone to justice,’ which is only used of trying and punishing the guilty, not securing the rights of the innocent.” The only context where the verb has positive connotations in a court setting is when the subject of the verb is an advocate who “takes someone’s side in a disputed matter.” The disputant doesn’t justify, nor does the judge; the advocate/lawyer justifies by speaking behalf of an accused.
When Prothro turns to the LXX, everything changes, in a “shocking” way. Dikaioo is used with personal objects in a positive sense, and “it is used this way in precisely the same contexts in which it had only negative senses everywhere else.” It comes to be used in parallel to “release” and “acquit,” and in opposition to verbs that connote punishment. Prothro suggests that the usage is consistent with the extra-biblical use of the verb in relation to advocacy: “If the translators understood judicial scenarios as basically a contention between two opposing parties and the judge’s act as authoritatively taking one side against the other to vindicate or acquit (cf. Deut 25,1), their use of dikaioo is, shall we say, justifiable.”Prothro is interested in the origin of Paul’s usage, and he persuasively argues that Paul doesn’t draw his language from, nor accommodate it to, standard pagan usage: “If dikaioo communicates that the divine Judge does something favorable for believers instead of condemning them, then the term’s origins do not lie in Greco-Roman jurisprudence.” Rather, he draws it from Jewish usage, where it appears “in theological discourse from the time of the LXX onward. God, Judge and Justifier over all disputes (l Kings 8,32; 2 Chron 6,23), ‘justifies’ Israel against her oppressors.” Specifically, Paul picks up the positive usage of dikaioo from LXX and other Jewish texts that use the verb positively in the setting of the divine courtroom, “in foro divino.“
Though Prothro focuses attention on the origins of Paul’s terminology, his analysis holds some significant theological implications. Protestant theologies of justification emphasize the forensic character of the term, a point that Prothro’s evidence abundantly supports. But the picture is often skewed by our understanding of courtroom proceedings. God the Judge doesn’t sit distantly, ensuring that the court proceeds smoothly; He doesn’t justify by a dispassionate sovereign act. Rather, He justifies as an Advocate who takes up the cause of one party in a dispute. This nudges us in the direction of seeing God’s justifying action taking place in the incarnation and history of Jesus, as He comes to the side of His people to vindicate them against their adversaries.
(James Prothro, “The Strange Case of Dikaioo in the Septuagint and Paul: The Oddity and Origins of Paul’s Talk of ‘Justification,’” Zeitschrift fur die Neutestamentliche Wissenschaft 107 : 48-69).