The Greek dikaioo is the verb form of dikaios (righteous), meaning to justify, to declare or pronounce righteous, to acquit. In the New Testament, it is the forensic declaration by God that a sinner is counted righteous on the basis of faith in Jesus Christ.
Dikaioo is the verb that encapsulates the Protestant Reformation's central insight: justification is a forensic declaration, not an infused moral transformation. When God 'dikaioo-s' (justifies) the ungodly (Romans 4:5), He does not make them morally perfect in that moment but pronounces a legal verdict — 'not guilty; righteous' — based on Christ's righteousness credited to their account. This is not legal fiction: it is made possible by Christ's actual bearing of sin at the cross. Romans 8:33–34 shows the forensic dynamic: 'Who will bring any charge against those whom God has chosen? It is God who justifies. Who then is the one who condemns?' The answer: no one, because the Judge has already spoken. James uses dikaioo in a complementary sense (2:21–24) — justified 'before others' by works that demonstrate the faith that God sees.