Westminster Larger Catechism in modern English question number seventy one:
Q. 71. How is justification an act of God’s free grace?
A. Although Christ, by His obedience and death, made a proper, real, and full satisfaction to God’s justice in the behalf of those that are justified—yet, inasmuch as God accepts the satisfaction from a surety (which he might have demanded of them), and did provide this surety, His own only Son, imputing His righteousness to them and requiring nothing from them for their justification except faith (which also is His gift), their justification is to them of free grace.