This has to be the best analogy I have ever read or heard of concerning the continuity of the covenant of grace based on the promise of God. Vos explains that the religion of the Old Testament and the religion of the New Testament are different parts of the same tree starting with the patriarchs (before the law), through the Mosaic economy (after the law is given), and then into the New Testament period (now that Christ has come).
“Paul has a profound insight into…patriarchal religion. His main contention with the Judaizers was that they insisted upon interpreting the patriarchal period on the basis of the Mosaic period. The reasoning [Gal. 3:15ff] is in substance as follows: through the diatheke with Abraham the relation between God and Israel was put on a foundation of promise and grace; this could not be subsequently changed, because the older arrangement remains regulative for later institutions [vs 15], and the law was by no less than 430 years later than the Abrahamic berith.
The revealed religion of the Old Testament resembles a tree whose root system and whose crown spread out widely, while the trunk of the tree confines the sap for a certain distance within a narrow channel. The patriarchal period corresponds to the root growth; the freely expanding crown to the revelation of the New Testament; and the relatively constricted form of the trunk to the period from Moses to Christ.”
-Geerhardus Vos, Biblical Theology, page 79