The Apostle Paul often contrasts the Mosaic law (works) with the Abrahamic promise (grace). He assumes that these two principles are fundamentally opposed: "For if the inheritance is based on law, it is no longer based on a promise" (Gal. 3:18). Since Paul tells us that the law did not invalidate the previous promise, how are we to understand the Mosaic Covenant?
First, we must note that the Mosaic Covenant is founded upon the Abrahamic promises. In Genesis 15:13-16, God promised Abraham that he would bring his descendants out of bondage, and in Exodus 3, it is the God of Abraham, Isaac, and Jacob that chooses Moses to lead his people out of Egypt. There is fundamental continuity. God's promises flow from Abraham through Moses to Christ.
But this covenant also functions on a typological level. For the nation finds itself in a covenant based on works, where future temporal blessings are entirely conditioned upon the people's obedience. Whereas the Abrahamic Covenant was ratified with God's oath, the covenant at Sinai is ratified by Israel's pledge of performance: "All that the Lord has spoken we will do" (Ex. 24:7). As such, law comes to the forefront of the nation's existence, though individual Israelites are still saved by grace (Ps. 51). (1) The history of Israel in the promised land relives Adam's history in the Garden, their disobedience resulting in forced exile. When Jeremiah prophesies of a new covenant that is unbreakable (gracious, irrevocable) in contrast to that which Israel has already broken (works, conditional), he is contrasting the consummation of the Abrahamic promises in Christ with the typological existence of the Israelite nation-just like Paul.
