This statement does not rest upon any express declaration of the Scriptures. It is, however, a concise and correct mode of asserting a plain Scriptural fact, namely, that God made to Adam a promise suspended upon a condition, and attached to disobedience a certain penalty. This is what in Scriptural language is meant by a covenant, and this is all that is meant by the term as here used. Although the word covenant is not used in Genesis, and does not elsewhere, in any clear passage, occur in reference to the transaction there recorded, yet inasmuch as the plan of salvation is constantly represented as a New Covenant, new, not merely in antithesis to that made at Sinai, but new in reference to all legal covenants whatever, it is plain that the Bible does represent the arrangement made with Adam as a truly federal transaction. The Scriptures know nothing of any other than two methods of attaining eternal life: the one that which demands perfect obedience, and the other that which demands faith. If the latter is called a covenant, the former is declared to be of the same nature. It is of great importance that the Scriptural form of presenting truth should be retained. Rationalism was introduced into the Church under the guise of a philosophical statement of the truths of the Bible free from the mere outward form in which the sacred writers, trained in Judaism, had presented them. On this ground the federal system, as it was called, was discarded. On the same ground the prophetic, priestly, and kingly offices of Christ were pronounced a cumbrous and unsatisfactory form under which to set forth his work as our Redeemer. And then the sacrificial character of his death, and all idea of atonement were rejected as mere Jewish drapery. Thus, by the theory of accommodation, every distinctive doctrine of the Scriptures was set aside, and Christianity reduced to Deism. It is, therefore, far more than a mere matter of method that is involved in adhering to the Scriptural form of presenting Scriptural truths.
God then did enter into a covenant with Adam. That covenant is sometimes called a covenant of life, because life was promised as the reward of obedience. Sometimes it is called the covenant of works, because works were the condition on which that promise was suspended, and because it is thus distinguished from the new covenant which promises life on condition of faith.
…The condition of the covenant made with Adam is said in the symbols of our church to be perfect obedience. That that statement is correct may be inferred (1.) From the nature of the case and from the general principles clearly revealed in the word of God. Such is the nature of God, and such the relation which He sustains to his moral creatures, that sin, the transgression of the divine law, must involve the destruction of the fellowship between man and his Creator, and the manifestation of the divine displeasure. The Apostle therefore says, that he who offends in one point, who breaks one precept of the law of God, is guilty of the whole. (2.) It is everywhere assumed in the Bible, that the condition of acceptance under the law is perfect obedience. “Cursed is every one who continueth not in all things written in the book of the law to do them.” This is not a peculiarity of the Mosaic economy, but a declaration of a principle which applies to all divine laws. (3.) The whole argument of the Apostle in his epistles to the Romans and to the Galatians, is founded on the assumption that the law demands perfect obedience. If that be not granted, his whole argument falls to the ground.
The specific command to Adam not to eat of a certain tree, was therefore not the only command he was required to obey. It was given simply to be the outward and visible test to determine whether he was willing to obey God in all things. Created holy, with all his affections pure, there was the more reason that the test of his obedience should be an outward and positive command; something wrong simply because it was forbidden, and not evil in its own nature. It would thus be seen that Adam obeyed for the sake of obeying. His obedience was more directly to God, and not to his own reason.
The question whether perpetual, as well as perfect obedience was the condition of the covenant made with Adam, is probably to be answered in the negative. It seems to be reasonable in itself and plainly implied in the Scriptures that all rational creatures have a definite period of probation. If faithful during that period they are confirmed in their integrity, and no longer exposed to the danger of apostasy. Thus we read of the angels who kept not their first estate, and of those who did. Those who remained faithful have continued in holiness and in the favour of God. It is therefore to be inferred that had Adam continued obedient during the period allotted to his probation, neither he nor any of his posterity would have been ever exposed to the danger of sinning.
—Charles Hodge (1797–1878), Systematic Theology (repr. Oak Harbor, WA: Logos Research Systems, Inc., 1997), 2.117–18, 119–120.