“He received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised. The purpose was to make him the father of all who believe without being circumcised, so that righteousness would be counted to them as well, and to make him the father of the circumcised who are not merely circumcised but who also walk in the footsteps of the faith that our father Abraham had before he was circumcised.” -Romans 4:11–12
God ordered the events this way not only so Abraham would be the father of all who have faith while still uncircumcised (Gentiles), but also so he would be the father of all them who are not merely circumcised but who also walk in the footsteps of the faith (True Israel).
“…For not all who are descended from Israel belong to Israel,” -Romans 9:6
Who then among the Israelites are the true Israel?
“For good news came to us (True Israel) just as to them (Ethnic Israel), but the message they heard did not benefit them, because they were not united by faith with those who listened.” -Hebrews 4:2
And,
“For neither circumcision counts for anything, nor uncircumcision, but a new creation. And as for all who walk by this rule, peace and mercy be upon them, and upon the Israel of God.” -Galatians 6:15–16
When Paul refers to “all who walk by this rule,” he is referring to those who by faith are “a new creation [in Christ].” See Romans 6:4.
“We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.” -Romans 6:4
Those who received the Word of God (about Christ’s death, burial, and resurrection) by faith the way Abraham received the Word of God (about the promise) by faith, are new creations. This is the rule (faith in Christ) the true Israel walks by, not circumcision and uncircumcision minus faith.
This leads us to an important conclusion, one that Paul will qualify and expound on going forward: Abraham’s faith and subsequent receipt of a promise prior to being circumcised shows that the covenant God made with him was real but temporary, as is the period of the Mosaic Law. It was an inadequate but preparatory substitute for the ultimate and complete covenant that would be confirmed in the incarnation and work of the Second Person of the Godhead himself, Jesus Christ.