Abraham’s saving faith was demonstrated before he was given the sign of circumcision. It was not the physical act of circumcision that saved, but his faith before the act. In Genesis 17 God gives Abraham a sign of his covenant: each male born in his household be circumcised on the eighth day. But Paul has cited Genesis 15:6 to show Abraham believed God, and that faith was “credited to him as righteousness.” This is after the second time God spoke to Abraham and conformed his covenant.
The three repetitions of the covenant are important since the progressively narrow the promised child from Abraham’s heir (Gen 12) to an heir coming from his own body (Gen 15) and then specifically Sarah’s child (Gen 17). At first, Abraham’s heir was an adopted child, Eliezer of Damascus, or perhaps Lot. After the second announcement, Abraham and Hagar have a child, Ishmael. After the third God specifically says the promised heir will be from Sarah and adds circumcision on the eighth day. Ishmael is circumcised (Gen 17:23), but he is thirteen at the time (Gen 16:16, Abraham is 86 when he is born, and Gen 17:1, he is 99 when the covenant is repeated a third time).
Circumcision was a “seal” of the righteous status Abraham had already received (Jewett, 317). Paul’s point is that Abraham was obedient to the sign of the covenant, but it was after his faith had been credited to him as righteousness. “Paul contends that circumcision served as a “seal” (σφραγίς) that confirms the validity of a reality already present, that is, righteousness through faith” (Jewett, 319).
Paul says God gave the sign when he did so that Abraham could be the father of all who believe, both the Jews and Gentiles. It was always possible Gentiles could become part of the people of God, although it was necessary for them to convert to Judaism. By the first century there was some debate whether Gentiles converting to Judaism were required to be circumcised (Galatians, for example, but also the Izates story in Josephus).
Paul’s point here is a radical re-reading of Abraham’s story since he argues Abraham can be the father of faithful Gentiles in their uncircumcision since Abraham himself was declared righteous before he was given the sign of circumcision. Circumcision was a key boundary marker in Second Temple Judaism, for Paul to suggest all Gentiles could be declared righteous like Abraham was without submitting to the sign of the covenant is radical indeed! (See, for example, my comments on Galatians 2 several years ago).
This means Abraham is not first the father of the Jews, but the father of Gentiles who are now being declared righteous by faith in Jesus (Barrett, Romans, 90-91).
In the same way, not ritual can impart any saving grace to a person, only through belief can a person be justified. Christian rituals such as baptism or communion are not intended to make a person right with God.