Justification is God's act of declaring sinners righteous — pardoning all their sins and accepting them as righteous — solely on the basis of Christ's righteousness imputed to them and received by faith alone. It is a legal verdict, complete the moment a sinner believes, and distinct from the inward renewal of sanctification.
“The whole nature of evangelical justification, consisting in the gratuitous pardon of sin and the imputation of righteousness, as the apostle expressly affirms, and the declaration of a believing sinner to be righteous thereon, as the word alone signifies, is utterly defeated by it.”
“(c) The Scriptures declare that our justification proceeds upon the ground of a perfect righteousness. "Christ is the end of the law for righteousness to every one that believeth." (Romans 10:3-9; 1 Corinthians 1:30.) The essence of pardon is that a man is forgiven without righteousness. The essence...”
“The apostle uses the present tense, γινώμεθα, we become righteous, because this justification is continuous.
We are introduced into a justified state.
In him, that is, in Christ.
It is by virtue of our union with Christ, and only as we are in him by faith, that we are righteous before God.
There...”
“In a word, the righteousness imputed being infinite, the justification grounded on it is at once complete, if it exists at all. (f.) The Papal idea that justification can be matured and carried on by inherent grace is inconsistent with God's nature and law. Suppose the believer reinstated in accepta...”
God's courtroom verdict over a believing sinner: "not guilty, and righteous in my Son." Christ's perfect record is credited to the believer the moment they trust him.
How is justification different from sanctification?
Justification changes our status before God (complete, instantaneous, by imputation). Sanctification changes our character (progressive, lifelong, by the Spirit's renewing work).