There is perhaps no better definition of the doctrine of justification, concise yet comprehensive, than that which the OPC confesses in the Westminster Larger Catechism (WLC) 70:
“Justification is an act of God’s free grace unto sinners, in which he pardoneth all their sins, accepteth and accounteth their persons righteous in his sight; not for anything wrought in them, or done by them, but only for the perfect obedience and full satisfaction of Christ, by God imputed to them, and received by faith alone.
In this definition, all the major elements of the biblical, Reformed doctrine are set forth. Justification is a forensic, judicial act of pardoning, accepting, and accounting, not a transformative work by which a sinner is made subjectively holy through an infusion of grace. According to this definition, justification is a blessing granted to sinners, those who have fallen short of God’s righteous requirements and stand condemned before him. In response to our sin, God, by his free grace, does two things for our justification: he pardons all our sins and accepts and accounts us righteous in his sight. God not only wipes away the guilt of sinners, but also credits righteousness to them. In justification, God declares that we are innocent of ever sinning against him and credits us with keeping his law perfectly. The ground for this great work, WLC 70 goes on to explain, is nothing that is wrought within us or done by us. Instead, its ground is the perfect obedience and full satisfaction of Christ, imputed to us. In other words, Christ’s perfect obedience to God’s law is credited to us, so that we stand before God as if we ourselves had kept that law perfectly. And Christ’s perfect sacrifice of atonement is imputed to us, so that we stand before God as if we had atoned for our lawbreaking. Finally, WLC 70 teaches that justification is received by faith alone. Not any work of obedience, but faith that receives and rests upon Christ, is the only instrument of justification.”
Taken from the OPC’s Report on Justification found here.