The two most important chapters in the Bible on covenant are Genesis 15 and Genesis 17. If you misunderstand these chapters, then you will fail to comprehend the covenantal language that pervades the rest of Scripture.
The most common error is to pit Genesis 15 against Genesis 17. Such an approach leads to the conclusion that Genesis 15 and 17 are speaking of different covenants.
Such a conclusion is unnecessary. There are differences between Genesis 15 and 17, but these differences should not be exaggerated. Often, passages that seem to be contradictory at first glance are, upon closer examination, found to complement one another. This is called harmonization.
Genesis 15 and 17 ought to be harmonized, not pitted against one another. These two chapters are speaking about the same covenant. The differences between these two chapters can be attributed to the fact that they are describing different aspects of the same covenant.
Genesis 15 highlights the promises of the covenant. God binds himself to fulfill the promises of the covenant. The dramatic ceremony illustrates God’s intense commitment to Abraham and his descendants.
If we only had Genesis 15, then we might conclude that God’s promises would be given to every single one of Abraham’s future offspring, without discrimination. However, this covenant is not perpetually unconditional. We see this in Genesis 17.
Genesis 17 highlights the succession of the covenant. How is the covenant passed down to future generations? Who inherits the promises of the covenant?
Genesis 17 reveals that the covenant is not unconditionally passed on to every single one of Abraham’s offspring without discrimination. God ordains circumcision to govern the succession of his covenant.
The introduction of circumcision reveals two foundational principles.
1) Abraham’s descendants must be circumcised to remain in the covenant. If they are not circumcised, then they are cut off from the covenant. They are no longer heirs of the promises. Not only is the uncircumcised individual cut off, but all of the subsequent offspring of the uncircumcised are cut off, as well. This is the pruning principle. The uncircumcised are pruned out of the covenant tree, regardless of their race.
2) Those unrelated to Abraham by blood must also be circumcised. Abraham’s household and other “strangers” who dwell among Abraham’s family must be circumcised. They are now heirs of the promises. Not only is each circumcised individual added to the covenant, but all of their subsequent offspring are added, as well. This is the grafting principle. The circumcised are grafted into the covenant tree, regardless of race.
The rest of the Bible reveals much more about the nature of this covenant, but Genesis 15 and 17 are the foundation. If you get these wrong, then you will misinterpret other covenantal texts.
Those who pit Genesis 15 against Genesis 17 misunderstand how the promises of God (Genesis 15) were mediated through circumcision (Genesis 17). Circumcision was the vehicle that caused God’s people to wax and wane through grafting and pruning.