In Gen 17:13, God tells Abraham that his whole household was to be circumcised והיתה בריתי בבשׂרכם לברית עולם (and my covenant will be in your flesh as an everlasting covenant). Yet, Paul strongly opposes Gentiles’ submitting to circumcision in connection with their membership in the Christian community (Galatians) and asserts that ὁ ἐν τῷ κρυπτῷ Ἰουδαῖος, καὶ {ὅτι} περιτομὴ καρδίας ἐν πνεύματι οὐ γράμματι (Rom 2:29; the Jew is one who is such inwardly, and [that] circumcision is of the heart by the Spirit, not by the letter). What then becomes of the בבשׂרכם []ברית עולם (Gen 17:13; everlasting covenant in your [= Abraham’s household’s] flesh)? It is precisely there because of the circumcision of Abraham’s messianic seed (Gal 3:16), ἐν ᾧ καὶ περιετμήθητε περιτομῇ ἀχειροποιήτῳ ἐν τῇ ἀπεκδύσει τοῦ σώματος τῆς σαρκός, ἐν τῇ περιτομῇ τοῦ Χριστοῦ (Col 2:11; in whom you also were circumcised with an unhandmade circumcision in the removal of the body of the flesh, in the circumcision of the Messiah; cf. Gal 3:23–29; Bede, Genesis, 284; Chrysostom, Hom. Col. 6 [NPNF1 13:285]; Cyril of Alexandria, Catena on Genesis [ACCOT 2:56]; Theodore of Mopsuestia, Colossians [ACCNT 9:32]).
In the case of Abraham and his household, it was an act of faith and belief in the promise of Yahweh regarding Abraham’s seed (and of the Messiah as well). In the case of Galatians, the act of circumcision is to satisfy a requirement for “belonging,” and is not an act of faith as such.
The “inner circumcision” is a confirmation of the understanding of what Abraham did with the “outer circumcision.” It’s always about faith that works, all day, all the time.
Kirk