“must be circumcised.” Circumcision was a requirement to be included in the covenant God made with Abraham. That is one reason that the Jews at the time of Paul were so insistent upon it. But the New Testament makes it clear that circumcision was not a requirement for Christians (Acts 15:1, 19-21; 1 Cor. 7:18; Gal. 6:12-15; Col. 3:11).
The reason Christians do not have to become circumcised to be “Abraham’s seed, and heirs of what was promised” is that we are completely in union with Jesus Christ. This means that in the eyes of God, we were crucified when Christ was crucified (Rom. 6:6), we died with Christ (Rom. 6:8), we were buried with Christ (Rom. 6:4); and we were made alive and then raised with Christ (Eph. 2:5-6). So too, we were circumcised with Christ, as Colossians says: “And through union with him you were also circumcised with a circumcision made without hands” (Col. 2:11 REV). But, much better than ordinary human circumcision, which removes only the unnecessary flesh of the foreskin, when Christians are “circumcised,” God removes our whole dead-flesh body! Christian “circumcision” is “a circumcision made without hands, consisting of the removal of the body of flesh, by the circumcision of Christ” (Col. 2:11 REV).
[For more on being in union with Jesus Christ, see commentary on Romans 6:3.]