Reformed covenant theology teaches that there is but one covenant of grace that God has made with His people in order to save them. This one covenant of grace is worked out in several subcovenants with Noah, Abraham, Moses, David, and Christ, and there are some differences in the outward form of the covenant of grace under each individual subcovenant. These differences should not be minimized. Nevertheless, the overarching unity of the one covenant of grace means that there is one way of salvation for everyone no matter the subcovenant in force at a particular time. That one way of salvation is faith in Christ alone (John 14:6). Evidence for the unity of the one covenant of grace is particularly clear in Romans 4, which tells us that Abraham, who lived under the Abrahamic covenant; David, who lived under the Mosaic covenant; and Christians, who live under the covenant in Christ are all justified by faith alone in God’s promised Messiah alone.
Differences between the subcovenants are particularly pronounced between the old covenant (the Mosaic covenant) and the new covenant (the covenant in Christ). The former has an extensive ceremonial law, civil law, and priestly system; whereas the latter does not, as Christ, who has fulfilled the Mosaic law, is the sole High Priest. This new covenant actually secures the full and final forgiveness for God’s people and grants us new hearts in accordance with the promise in Jeremiah 31:31–34, part of which is quoted in today’s passage. Since this new covenant is so much better, why would anyone want to go back to the old one? In fact, to do so is to incur divine judgment, as the epistle to the Hebrews argues.
The new covenant is necessary, Hebrews 8:8 tells us, because God found fault with the people. Our Creator never intended the old covenant to bring the blessings we have under the new covenant, though the old covenant saints possessed the benefits of the new covenant, albeit to a lesser degree than we do. Still, the inadequacy of the old covenant order is tied primarily to human sin; the fault is with sinners, not with the covenant and its law (Rom. 7:7–12; Gal. 3:21).
In and through the new covenant, we get what the subcovenants of the covenant of grace hoped for. The new covenant, Hebrews 8:10 reveals, is the means by which God’s promise to be God to Abraham is accomplished (see Gen. 17:7). Without the new covenant, the covenant of grace goes unfulfilled.