How is justification an act of God’ s free grace?
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; (Rom. 5:8–10,19) 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, (1 Tim. 2:5–6, Heb. 10:10, Matt. 20:28, Dan. 9:24,26, Isa. 53:4–6,10–12, Heb. 7:22, Rom. 8:32, 1 Pet. 1:18–19) imputing his righteousness to them, (2 Cor. 5:21) and requiring nothing of them for their justification but faith, (Rom. 3:24–25) which also is his gift, (Eph. 2:8) their justification is to them of free grace. (Eph. 1:7)
The Westminster Larger Catechism: With Scripture Proofs. (Oak Harbor, WA: Logos Research Systems, Inc., 1996).