Paul does not describe the shift from the Old to New as a simple shift from external to internal…Paul’s letter is not primarily about individual soteriology, but about the union of Jews and Gentiles in the one new man, Jesus the Christ, and the coming of a new creation through His death and resurrection…All those who share the faith of Abraham are “sons of God” (v. 26), that is, true Israelites (cf. Exod. 4:23)…they are all heirs of the inheritance promised to Abraham, the promise of the Spirit (vv. 28-29)…Baptism into Christ and being clothed with Christ is thus all about incorporation into membership in this new body, the body that is “one in Christ Jesus” (v. 28), the community of those who “are Christ’s” (v. 29)…Circumcision distinguished between Jew and Gentile, and also between male and female. In the New Covenant, baptism is applied indiscriminately to all who believe – whether Jew or Gentile, slave or free, male or female. Baptism thus symbolizes and enacts the union of Jew and Gentile int he church, ritually marking all the baptized as sons of Abraham.
Peter J. Leithart, The Baptized Body (Moscow, ID: Canon Press, 2007), 45-46.