Could later commands cancel the promise?

The short answer Settled

No. The man who refused circumcision was cut off from his people, but the covenant stood. The covenant at Sinai was truly conditional, but it was a separate covenant made 430 years later, and Paul says a later covenant cannot cancel or add to one God had already confirmed.

Circumcision: what was broken?

Genesis 17 is the first place God's promises to Abraham lay a duty on Abraham's side. God commands circumcision and says of the man who refuses it, "that person shall be cut off from his people; he has broken My covenant" (Genesis 17:14). That is a real command with a severe penalty, and "broken" is the right word.

But the sentence says what is removed, and it is the man. He is cut off "from his people" (Genesis 17:14), which assumes the people are still there. The rule is given "in your generations" (Genesis 17:12), to be applied again and again; if the first offender ended the covenant, there would be no covenant people left for the next offender to be cut off from. The verse just before the penalty calls this same covenant "an everlasting covenant" (Genesis 17:13). And circumcision itself is called "a sign of the covenant between Me and you" (Genesis 17:11). A sign can be refused, and refusing it is a real offense, but that is not the same thing as the covenant ceasing to exist.

Paul reads the order the same way. Abraham was counted righteous before he was circumcised, and afterward "he received the sign of circumcision, a seal of the righteousness of the faith which he had while still uncircumcised" (Romans 4:11). The promise came first; the sign was added to it.

Sinai: a second covenant with conditions

The covenant at Sinai reads differently from the one with Abraham. God says, "if you will indeed obey My voice and keep My covenant, then you shall be a special treasure to Me" (Exodus 19:5), and the people answer, "All that the LORD has spoken we will do" (Exodus 19:8). Deuteronomy 28 sets blessing and curse side by side: "if you diligently obey the voice of the LORD your God" (Deuteronomy 28:1), and "if you do not obey the voice of the LORD your God" (Deuteronomy 28:15). Nothing like that is attached to the promise in Genesis 15 or 17. There, only one party spoke, and no verse says what happens to the grant if Abraham's descendants disobey.

Paul takes up the relationship between the two directly. He states a rule: "Though it is only a man's covenant, yet if it is confirmed, no one annuls or adds to it" (Galatians 3:15). Then he applies it: "the law, which was four hundred and thirty years later, cannot annul the covenant that was confirmed before by God in Christ, that it should make the promise of no effect" (Galatians 3:17). He treats the two as different grounds of inheritance: "For if the inheritance is of the law, it is no longer of promise; but God gave it to Abraham by promise" (Galatians 3:18).

The promise was already in force before Sinai existed. In Egypt, "God heard their groaning, and God remembered His covenant with Abraham, with Isaac, and with Jacob" (Exodus 2:24). The covenant with Abraham is the reason God acted, and that action led to Sinai.

The usual objection

The objection says the two covenants are one arrangement. God promised Abraham the land, and Sinai supplied the terms on which it was held. Israel broke the terms, so the promise is void. Some add that Genesis 17 itself shows the Abrahamic covenant could be broken.

Much of that is true. The two covenants are connected: same God, same people, same land. The conditions at Sinai are real, and Israel failed them. The word "broken" in Genesis 17 is strong. But connected is not the same as identical. For the argument to work, the Sinai covenant would have to amend the promise, adding conditions to it and so becoming able to cancel it. Cancelling and adding are exactly the two things Paul says no one does to a covenant already confirmed.

The words behind it