In Genesis 15:4 – 6, Abraham is promised an heir and an uncountable number of descendants. “Abraham believed God and it was accounted to him as righteousness.” Circumcision began at least 14 years later, in chapter 17. Chapter 15 is before Ishmael. In chapter 17, Ishmael is 13. The Genesis 15 scene is recalled in Romans 4:3, 9 – 12, and 16 – 22, making the point that Abraham’s faith was accounted as righteousness before he was circumcised. Therefore, circumcision is not necessary to be acceptable since Abraham was uncircumcised when he was first said to be righteous, as well as after being circumcised.
Genesis 17:2 – 4 repeats this promise, but again before God introduced the concept of circumcision. Two more promises are given in 17:7 – 8, that God will be the God of Abraham’s descendants and that those descendants will inherit the land of Canaan in perpetuity. The sign of this covenant (promise) was to be circumcision. Some include the promise in 17:2 – 4 in the sign of circumcision. Others do not because the “uncountable descendants” promise was a repeat of 15:4 – 6, so they count only the latter two promises in the sign of circumcision.
Romans 4:17 repeats the Genesis 17:5 account of the “uncountable descendants” promise, but separates it from circumcision by insisting that Abraham is the “father” of all the faithful, whether or not the faithful person has been circumcised.
Faithful Gentiles were not circumcised as a point of some importance (Galatians 2:3), although faithful Jews were (Acts 16:3). Whether to circumcise faithful Gentiles was a matter of some contention (Acts 15:1, 15:5) and was addressed directly by Paul (Galatians 5:2 – 6, 6:12 – 13). The faithful are described as “the circumcision” (Philippians 3:2 – 4) sarcastically, in opposition to those who believed circumcision to be necessary for all the faithful. The faithful are “circumcised with a circumcision made without hands.” (Colossians 2:11) Paul specifically instructed the uncircumcised not to be circumcised. (1 Corinthians 7:18 – 19)
Therefore, the “countless descendants” promise in Genesis 17 is not part of the other two promises for which circumcision is the sign. However, since the promise in Genesis 17:7 – 8 includes the adjective, everlasting, it seems that a promise remains for faithful Israelites (circumcised, presumably) to be able to claim the Promised Land. The present Israelis are not in this group because they fail to follow Jesus as the faithful will do.