Westminster Larger Catechism
Q. 71. How is justification an act of God’s free grace?
A. Although Christ, by his obedience and death, did make a proper, real, and full satisfaction to God’s justice in the behalf of them that are justified;[291] yet in as much as God accepteth the satisfaction from a surety, which he might have demanded of them, and did provide this surety, his own only Son,[292] imputing his righteousness to them,[293] and requiring nothing of them for their justification but faith,[294] which also is his gift,[295] their justification is to them of free grace.[296]
Daily Westminster 