Following up on the last post: if eternal life was indeed promised in the covenant of works, then is there no difference between what Adam could have won for his posterity and what Jesus Christ actually purchased for those in him? Again, John Colquhoun:
It ought here, however, to be observed, that though the eternal life in heaven, which was promised in the covenant of works, was the same in its nature, with that which is promised in the covenant of grace; yet, in several respects, it would have been inferior to it. — I shall mention a few of them.
1. The title of Adam in innocence, to eternal life, could not have been confirmed, in the adorable person, and stupendous death, of the Son of God incarnate; nor could the charter of his right to it, have been what it now is, to every true believer in Jesus, — a new testament, or new covenant in his blood. Adam was to have had good security for life, namely, the covenant of works fulfilled; but the true Christian, has a far more glorious charter; — the everlasting covenant of Grace, written with blood, the infinitely precious blood, of Jesus the only begotten of the Father.
2. Upright Adam, could not have seen in heaven, what the glorified saint will now behold, the incarnate Lamb, the Lamb, as if it had been slain. He was to have enjoyed bright discoveries of God Father, Son, and Holy Spirit: but he could not have been blessed with the beatifical vision of the eternal Son, in human nature; — that immaculate Lamb, who is ten thousand times brighter than our meridian sun, and will to all eternity, continue to be the light of the heavenly temple. He could have beheld Jehovah sitting upon the throne; but not, the Lamb in the midst of the throne. He could have contemplated the only-begotten Son, in heaven, and in the bosom of the Father, but not, in the human nature; not, as his near kinsman, his brother, who for him and for his salvation, was dead, but is now alive, and liveth forevermore. He could have had none of those astonishing, and transporting, manifestations of the glory of Jehovah, in the face of Jesus Christ; none of those delightful discoveries of his perfections, and purposes, in the glorious work of redemption, which, in heaven, are and shall be enjoyed by the ransomed of the Lord.
3. Again, Innocent Adam, could indeed have praised Him who sitteth on the throne, as the Creator and Preserver of all things; but he could not have joined, in this transporting anthem: "Unto him that loved us, and washed us from our sins in his own blood; and hath made us kings and priests unto God and his Father: to him be glory, and dominion for ever and ever, Amen" (Rev. 1.5, 6).
4. Adam could have dwelt in heaven, as the creature, the servant, and the friend, of God the Son; but the redeemed present themselves there, as his brethren and sisters, his spouse, his members (Eph. 5.30; John 14.19), and his spiritual seed, the fruit of the travail of his soul. He could not have been so early allied, nor so intimately related, to the only-begotten Son, as they are honoured to be.
5. Upright Adam, could have sat down before the celestial throne, arrayed in the garment of his own righteousness; but not, as invested with that spotless, that best robe, the immaculate righteousness of the incarnate Redeemer, with which, as their garment of salvation, his ransomed are adorned,
6. In a word, Adam's enjoyment of eternal life, could not have been sweetened, by his remembrance of any sad experience, that he formerly had had of sin, or of misery, or of sorrow; as will be that of the redeemed from among men. The relish, which the saints shall have, of the pleasures that are at the right hand of God, will, after their bitter experience of sin, sorrow, sickness, and pain, be higher than Adam's could have been, who is supposed never to have known, what it was to experience any of those evils. [emphasis in the original]
Treatise of the Covenant of Works, John Colquhoun, pages 76-78