From the Shorter Catechism

Q. 33. What is justification?

A. Justification is an act of God’s free grace, wherein he pardoneth all our sins, and accepteth us as righteous in his sight, only for the righteousness of Christ imputed to us, and received by faith alone. Rom. 3:24-25; Rom. 4:6-8; 2 Cor. 5:19, 21; Rom. 5:17-19; Gal. 2:16; Phil. 3:9.