WCF 16.5.
We cannot by our best works merit pardon of sin, or eternal life at the hand of God, by reason of the great disproportion that is between them and the glory to come; and the infinite distance that is between us and God, whom, by them, we can neither profit, nor satisfy for the debt of our former sins, (Rom. 3:20, Rom. 4:2, 4, 6, Eph. 2:8–9, Tit. 3:5–7, Rom. 8:18, Ps. 16:2, Job 22:2–3, Job 35:7–8) but when we have done all we can, we have done but our duty, and are unprofitable servants: (Luke 17:10) and because, as they are good, they proceed from His Spirit; (Gal. 5:22–23) and as they are wrought by us, they are defiled, and mixed with so much weakness and imperfection, that they cannot endure the severity of God’s judgment. (Isa. 64:6, Gal. 5:17, Rom. 7:15, 18, Ps. 143:2, Ps. 130:3)