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;^[1] yet inasmuch as God accepteth the satisfaction from a surety, which he might have demanded of them, and did provide this surety, his own only Son,^[2] imputing his righteousness to them,^[3] and requiring nothing of them for their justification but faith,^[4] which also is his gift,^[5] their justification is to them of free grace.^[6]