Q.33. What is justification?
A. Justification is an act of God’s free grace, wherein he pardons all our sins, and accepts us as righteous in his sight, only for the righteousness of Christ imputed to us, and received by faith alone. Westminster Shorter Catechism
Those whom God effectually calls, he also freely justifies, not by infusing righteousness into them, but by pardoning their sins, and by accounting and accepting their persons as righteous; not for anything wrought in them, or done by them, but for Christ’s sake alone; not by imputing faith itself, the act of believing, or any other evangelical obedience to them, as their righteousness; but by imputing Christ’s active obedience unto the whole law, and passive obedience in his death for their whole and sole righteousness by faith, which faith they have not of themselves; it is the gift of God.
From the 1689 London Baptist Confession of Faith