That eternal life in heaven, was promised to mankind, in the first covenant, may be evinced by the following arguments:
1. The life, which the second Adam, by his surety-righteousness, merited for his spiritual seed, is the same that the first Adam had forfeited, by his disobedience. Now that was life eternal. If "he who believeth, hath eternal life," and, if "the just shall live" an eternal life "by faith," "The man which doeth those things, shall live by them" (Rom. 10.5); that is, shall also live the same eternal life. Hence too, are these words of the apostle Paul : "That no man is justified by the law in the sight of God, it is evident; for the just shall live by faith. And the law is not of faith; but the man that doeth them, shall live in them" (Gal. 3.11-12). That eternal life, then, which is received by faith, is here represented, as in substance the same with this, which, by the law, is connected with the perfect fulfillment of all its demands.
2. Our blessed Lord, when proposing the covenant of works, to a young man who was a legalist, told him, that he should have eternal life, as the performance of the promise of it, upon his fulfilling of the condition. "Behold one came and said unto him, good Master, what good thing shall I do, that I may have eternal life? And he said unto him, — If thou wilt enter into life, keep the commandments (Matt. 19.16-17). On another occasion, he returned a similar answer to a lawyer, who put a similar question to him: "He said unto him, thou hast answered right; this do and thou shalt live "(Luke 10.28), that is,— shalt live an eternal life. It was eternal life, then, which, in that covenant, was promised to Adam, upon condition of his perfect obedience to the law.
3. If for the breach of that covenant, Adam and all his natural posterity in him, were condemned to eternal death (Rom. 6.23; Matt. 25.46); it is requisite, according to the rules of remunerative justice, and the infinite equity of the Divine procedure, that if the condition of life had been fulfilled, they should have been adjudged to eternal life. Was the penalty of death eternal in the infernal world, annexed to the transgression even of a positive precept ? We may warrantably conclude, that life everlasting in the heavenly world, was included in the promise.
4. Reason also suggests, that, his all-bountiful Lord would promise to the man, as the reward of his obedience, a better life, than that which he already possessed; — that, as the earthly paradise, was adapted principally to promote the happiness of his body, there would be another, and a future state and place, fitted to afford such spiritual felicity, as should chiefly correspond to the spiritual, and heavenly, nature of his soul; — and that, after his present state of service and of trial, there would very likely be a future one, of reward and of enjoyment.
5. The divine appointment of the tree of life, as a symbol and seal of that covenant, plainly hinted that the promise of it, was a promise of a better, and even of an eternal life. — The tree of life signified and sealed to Adam, that upon the ground of his finished obedience, he should be confirmed in life, and should live for ever. It appears from the following words, that he so understood the design of it: "And now lest he put forth his hand, and take also of the tree of life, and eat, and live forever; therefore the Lord God sent him forth from the garden of Eden. And he planted at the east of the garden of Eden, Cherubim and a flaming sword, which turned every way, to keep the way of the tree of life" (Gen. 3.22-24).
6. Christ the second Adam, by his unsmiling obedience, merited for his spiritual offspring, that very life, which the law could not, because of the sinfulness of their nature, afford to them .— " For what the law could not do, in that it was weak through the flesh, God sending his own Son, in the likeness of sinful flesh, and for sin, condemned sin in the flesh: that the righteousness of the law might be fulfilled in us" (Rom. 8.3-4), &c. The law or commandment, under the form of a covenant of works, "was," we are told, "ordained to life;" ordained, to confer on them who should keep it, eternal life in heaven. "If thou wilt enter into life, keep the commandments" (Matt. 19.17). The inability of the law, therefore, to afford eternal life now, arises merely from the sinner's inability, to afford that perfect obedience, which it originally required, and still requires as the ground of a title to life. If sin had not been committed, the law as a covenant of works, could have still conducted men to that everlasting life, which Christ the last Adam, confers upon his children.
7. In the last place, Though justification, in which men are confessedly declared righteous, and entitled to eternal life, is, in the sacred records, affirmed to be altogether impossible now, by the works of the law; yet, it is nowhere suggested, that this proceeds from any other cause, than the sinner's own inability, to answer the just demands of the violated law (Rom. 3.19-20). It is nowhere hinted, that the reason why a man cannot be justified, or procure a title to eternal life, by the deeds of the law, is, that the law never had a promise of eternal life; but, that — "by the law, is the knowledge of sin;" and, that — "all the world have become guilty before God." Thus it is manifest, that eternal life, was promised in the covenant of works.
[emphasis in the original]
Treatise of the Covenant of Works, John Colquhoun, pages 73-76