“For encouragement to regenerate persons. Though you fail in your obedience, and cannot keep the moral law exactly, yet be not discouraged.

What comfort may be given to a regenerate person under the failures and imperfections of his obedience?

That a believer is not under the covenant of works, but under the covenant of grace. The covenant of works requires perfect, personal, perpetual obedience; but in the covenant of grace, God will make some abatements; he will accept less than he required in the covenant of works. (1) In the covenant of works God required perfection of degrees; in the covenant of grace he accepts perfection of parts. There he required perfect working, here he accepts sincere believing. In the covenant of works, God required us to live without sin; in the covenant of grace he accepts of our combat with sin. (2) Though a Christian cannot, in his own person, perform all God’s commandments; yet Christ, as his Surety, and in his stead, has fulfilled the law for him: and God accepts of Christ’s obedience, which is perfect, to satisfy for that obedience which is imperfect. Christ being made a curse for believers, all the curses of the law have their sting pulled out. (3) Though a Christian cannot keep the commands of God to satisfaction, yet he may to approbation.

How is that?

(1) He gives his full assent and consent to the law of God. ‘The law is holy and just:’ there was assent in the judgement. Rom 7:12. ‘I consent unto the law;’ there was consent in the will. Rom 7:16.

(2) A Christian mourns that he cannot keep the commandments fully. When he fails he weeps; he is not angry with the law because it is so strict but he is angry with himself because he is so deficient.

(3) He takes a sweet complacent delight in the law. ‘I delight in the law of God after the inward man.’ Rom 7:22. Greek: ‘I take pleasure in it.’ ‘O! how love I thy law.’ Psa 119:97. Though a Christian cannot keep God’s law, yet he loves his law; though he cannot serve God perfectly, yet he serves him willingly.

(4) It is his cordial desire to walk in all God’s commands. ‘O that my ways were directed to keep thy statutes.’ Psa 119:5. Though his strength fails, yet his pulse beats.

(5) He really endeavours to obey God’s law perfectly; and wherein he comes short he runs to Christ’s blood to supply his defects. This cordial desire, and real endeavour, God esteems as perfect obedience. ‘If there be a willing mind, it is accepted.’ 2 Cor 8:12. ‘Let me hear thy voice, for sweet is thy voice.’ Cant 2:14. Though the prayers of the righteous are mixed with sin, yet God sees they would pray better. He picks out the weeds from the flowers; he sees the faith and bears with the failing. The saints’ obedience, though short of legal perfection, yet having sincerity in it, and Christ’s merits mixed with it, finds gracious acceptance. When the Lord sees endeavours after perfect obedience, he takes it well at our hands; as a father who receives a letter from his child, though there be blots in it, and false spellings, takes all in good part. Oh! what blotting are there in our holy things; but God is pleased to take all in good part. He says, ‘It is my child, and he would do better if he could; I will accept it.’ “

Thomas Watson, Westminster Shorter Catechism Project, Body of Divinity Contained in Sermons Upon the the Assembly’s Catechism