Besides this evangelical character which unquestionably belongs to the Mosaic covenant, it is presented in two other aspects in the Word of God. First, it was a national covenant with the Hebrew people. In this view the parties were God and the people of Israel; the promise was national security and prosperity; the condition was the obedience of the people as a nation to the Mosaic law; and the mediator was Moses. In this aspect it was a legal covenant. It said. “Do this and live.” Secondly, it contained, as does also the New Testament, a renewed proclamation of the original covenant of works. It is as true now as in the days of Adam, it always has been and always must be true, that rational creatures who perfectly obey the law of God are blessed in the enjoyment of his favour; and that those who sin are subject to his wrath and curse. Our Lord assured the young man who came to Him for instruction that if he kept the commandments he should live. And Paul says (Rom. ii. 6) that God will render to every man according to his deeds; tribulation and anguish upon every soul of man that doeth evil; but glory, honour, and peace to every man who worketh good. This arises from the relation of intelligent creatures to God. It is in fact nothing but a declaration of the eternal and immutable principles of justice. If a man rejects or neglects the gospel, these are the principles, as Paul teaches in the opening chapters of his Epistle to the Romans, according to which he will be judged. If he will not be under grace, if he will not accede to the method of salvation by grace, he is of necessity under the law.
These different aspects under which the Mosaic economy is presented account for the apparently inconsistent way in which it in spoken of in the New Testament. (1.) When viewed in relation to the people of God before the advent, it is represented as divine and obligatory. (2.) When viewed in relation to the state of the Church after the advent, it is declared to be obsolete. It is represented as the lifeless husk from which the living kernel and germ have been extracted, a body from which the soul has departed. (3.) When viewed according to its true import and design as a preparatory dispensation of the covenant of grace, it is spoken of as teaching the same gospel, the same method of salvation as that which the Apostles themselves preached. (4.) When viewed, in the light in which it was regarded by those who rejected the gospel, as a mere legal system, it was declared to be a ministration of death and condemnation. (2 Cor. iii. 6-18.) (5.) And when contrasted with the new or Christian economy, as a different mode of revealing the same covenant, it is spoken of as a state of tutelage and bondage, far different from the freedom and filial spirit of the dispensation under which we now live.
-Charles Hodge, Systematic Theology, vol. 2, p. 375
Hodge’s statement eloquently sets forth the traditional Reformed understanding of the Mosaic covenant as being a part of the Covenant of Grace while at the same time setting forth a proclamation of the Covenant of Works. This is similar to the argument I make HERE (with Samuel Bolton, Matthew Henry, and Herman Ridderbos), that the statement in Romans 8:2 “the law of the Spirit of life has set you free in Christ Jesus from the law of sin and death,” refers to one law. The Law is a law of sin and death to those outside Christ, but it is the law of the Spirit of life for those in Christ. It is our orientation toward the law that determines how we will relate to it.