Thomas Boston: Law-keeping will never avail as a ground for justifying righteousness before God

Thomas Boston explains, as summarized by Brian Schwertley, "why personal law-keeping or covenant faithfulness can have nothing to do with our justification":

1. Thy obedience must be perfect, in respect of the principle of it; that is, thy soul, the principle of action, must be perfectly pure, and altogether without sin. For the law requires all moral perfection; not only actual, but habitual: and so condemns original sin; impurity of nature as well as of actions. Now, if thou canst bring this to pass, thou shalt be able to answer that question of Solomon’s, so as never one of Adam’s posterity could yet answer it, ‘Who can say, I have made my heart clean?’ Prov. xx. 9. But if thou canst not, the very want of this perfection is sin, and so lays thee open to the curse, and cuts thee off from life. Yea, it makes all thine actions, even thy best actions, sinful: ‘For who can bring a clean thing out of an unclean?’ Job xiv. 4. And dost thou think by sin, to help thyself out of sin and misery? 2. Thy obedience must also be perfect in parts. It must be as broad as the whole law of God: if thou lackest one thing thou art undone; for the law denounces the curse on him that continues not in every thing written therein, Gal 3:10. Thou must give internal and external obedience to the whole law; keep all the commands in heart and life. If thou breakest any one of them, that will ensure thy ruin. A vain thought, or idle word, will still shut thee up under the curse. 3. It must be perfect in respect of degrees; as was the obedience of Adam, while he stood in his innocence. This the law requires, and will accept of no less, Matt. Xxii. 37, ‘thou shalt love the Lord thy God, with all thy heart, and with all thy soul, and with all thy mind.’ If one degree of that love, required by the law, be wanting; if each part of thy obedience be not brought up to the greatest height commanded; that want is a breach of the law, and so leaves then still under the curse. A man may bring as many buckets of water to a house that is on fire, as he is able to carry; and yet it may be consumed, and will be so, if he bring not as many as will quench the fire. Even so, although thou shouldest do what thou art able, in keeping the commands, if thou fail in the least degree of obedience, which the law enjoins, thou art certainly ruined for ever; unless thou take hold of Christ, renouncing all thy righteousness, as filthy rags. See Rom. x. 5; Gal. iii. 10. Lastly, It must be perpetual, as the man Christ’s obedience was, who always did the things which pleased the Father; for the tenor of the law is, “Cursed is he that continueth not in all things written in the law, to do them.” Hence, though Adam’s obedience was, for awhile, absolutely perfect; yet because at length he tripped in one point, viz. in eating the forbidden fruit, he fell under the curse of the law. If a man were to live a dutiful subject to his prince, till the close of his days, and then conspire against him, he must die for his treason. Even so, though thou shouldst all the time of thy life, live in perfect obedience to the law of God, and yet at the hour of death only entertain a vain thought, or pronounce an idle word, that idle word, or vain thought, would blot out all thy former righteousness, and ruin thee; namely, in this way in which thou art seeking to recover thyself.

Now such is the obedience which thou must perform, if thou wouldst recover thyself in the way of the law. (Thomas Boston, Human Nature in Its Fourfold State [Philadelphia: Presbyterian Board of Publication, n.d.], 120-121; quoted by Brian Schwertley in his refutation of the Auburn Avenue theology)