WLC Q.71: How is justification an act of God’s free grace?
Although Christ, by his obedience and death, did make a proper, real, and fully 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)
Christ, by His obedience and death, did fully discharge the debt of all those that are thus justified, and did make a proper, real, and full satisfaction to His Father’s justice in their behalf. (Rom. 5:8–10, 19, 1 Tim. 2:5–6, Heb. 10:10, 14, Dan. 9:24, 26, Isa. 53:4–6, 10–12) Yet, in as much as He was given by the Father for them; (Rom. 8:32) and His obedience and satisfaction accepted in their stead; (2 Cor. 5:21, Matt. 3:17, Eph. 5:2) and both, freely, not for any thing in them; their justification is only of free grace; (Rom. 3:24, Eph. 1:7) that both the exact justice, and rich grace of God might be glorified in the justification of sinners. (Rom. 3:26, Eph. 2:7)