A twofold witness

Conscience and Scripture both testify to the reality of the covenant of works. Consider these brilliant extracts from Cocceius and Mastricht, helpfully provided and introduced by Heinrich Heppe in his Reformed Dogmatics. (Formatting and punctuation edited here for readability, and Heppe’s copying out of Scripture texts suppressed.)

Cocceius (Summ. theol. XXII, 20-21) explains how man is taught by his conscience about the covenant of works originally ordained by God. Conscience witnesses to man that he “who has preserved the image of God and has done righteously in accordance with it “has” a covenant with God, provided that there is (1) “no offense between him and God but (2) the peace by which benefits are possessed in security; but also (3) that he is righteous, i.e. has a right to God’s friendship and communion and to ask and expect of God what it is just, right and holy to expect of God”. Conscience bears witness that “God cannot put off those who seek Himself or refuse to satisfy and fulfill a right and holy desire”. Could it be assumed that God wished not to be found of them for their enjoyment, it would follow that it is wrong to seek and desire God. But if it is wrong, God “is not man’s good and cannot make man happy in Himself.” So it would have to be the case instead that “man’s good” and its “end” “are things created by God; for man’s good is man’s end”. Thus it follows (XII, 22) “that he who does what conscience dictates has exousia and power to call God his God and to glory in Him as his good”.

So conscience itself is a witness to the reality of the covenant of works. But we are not left with the light of nature in this regard: God has spoken in His word as well.

What is thus guaranteed to man by the voice of his conscience is confirmed for him by the witness of H. Scripture�From H. Scripture the fact is proved as follows (Mastricht III, xii, 23), that God has made a covenant with Adam:
“(1) It specifically (rhtos) states in Hosea 6:7: ‘they like Adam have transgressed the convenant [sic]; there have they dealt treacherously against me’; cf. Job 31:33 (if after the manner of Adam (man) I covered my transgressions by hiding mine iniquity in my bosom) where the best translators (Vulgate, Tigurinus, Pagninus, Castalio, Belgae and others) take [Hebrew] proprie, not appellative, though I admit there are not wanting those who prefer the appellative sense.
(2) The Apostle, Gal. 4:24 ,mentions a double covenant, the former of which is ‘by the works of the law’, 2:16, demanding most punctilious obedience, 3:10, by means of which no can ever obtain everlasting life, 2:16; 3:2, under which we all were until the covenant of faith, 3:23, and are, as long as we live as children of the flesh, 3:22,29, which only begets to slavery, 3:24; Heb. 2:14-15. And this is the very thing which we call the covenant of works, subsequently; as the result of the faith of the Gospel. If you say the apostle is speaking of a covenant not in Paradise, but the covenant at Sinai, the answer is easy, that the Apostle is speaking of the covenant in Paradise so far as it is re-enacted and renewed with Israel at Sinai in the Decalogue, which contained the proof of the covenant of works.
(3) Synonyms of the covenant of works are extant in the NT, Rom. 3:27; Gal. 2:16. Moreover what is the law of works but the covenant of works? What is law simpliciter as opposed to grace? Rom. 8:3; what, I say, if not the legal covenant? Because we are said to be not under the law but under grace, Rom. 6:14-15; 4:16, what is that but that we are not under the covenant of the law? At least for us these are plainly synonymous.
(4) We have previously in the exegetical section shewn that all the essentials of the covenant of works are contained in the first publication of it, Gen. 2:17.
(5) To very many heads of the Christian religion, e.g., the propagation of original corruption, the satisfaction of Christ and his subjection to divine law, Rom 8:3-4; Gal. 3:13; 4:4-5, we can scarcely give suitable satisfaction, if the covenant of works be denied”.

Item 2 of course has deep interest in its mention of the doctrine of the republication of the covenant of works in the Mosaic administration of the covenant of grace. (See also Thomas Boston on the subject.) Item 3 is a fascinating line of argument, and something to keep in mind to follow up in more detail. But note item 5. The covenant of works is to be accepted as a Scriptural doctrine because of its explanative power. By upholding the doctrine of the covenant of works we are able to give full weight and due credit to many representations of Scripture which we should otherwise be tempted to underplay or deny, if we care about a harmonious doctrinal system, or accept as flat contradictions if we don’t. This gives the covenant of works the force of due and necessary consequence, in addition to the direct testimony of Scripture and conscience.