Galatians 3:16-18 denies that the Mosaic covenant annulled the promises of the Abrahamic and proceeds to argue the Christian gospel on the basis of the Abrahamic covenant against the idea of salvation by keeping the Mosaic law.
Yet, when the covenant was initially made with Abraham, God gave circumcision as the sign. Circumcision was given to Abraham (Genesis 17), not Moses (Exodus 34). Yet, in Paul especially, circumcision is often associated and even identified with the law of Moses in distinction to salvation by faith.
Paul seems emphatic that circumcision is not necessary and that trusting in circumcision is dangerous. It is easy to see why Paul would warn against trusting in works of the law given through Moses, but it is easy to sympathize with someone who feared the prospect of uncircumcision.
Genesis 17:14 Any uncircumcised male who is not circumcised in the flesh of his foreskin shall be cut off from his people; he has broken my covenant.
(Again, speaking to Abraham, not Moses).
Paul is not alone in his interpretation. It is also the verdict of the Jerusalem council in Acts 15, affirmed by none other than Peter himself. In Acts 15, they associate circumcision with a "custom of Moses" (v. 1 and 5 by the pro-circumcision men from Judea and Pharisees, and again in v. 10 when Peter indirectly refers to it as a yoke).
The bow placed in the sky in the Noahic covenant remains because the covenant with creation remains, summer and winter, seedtime and harvest. So it is reasonable to believe that even very ancient covenant signs that came before the advent of Christ could still be in effect if their corresponding covenants remain in effect.
Why is circumcision then associated with Moses and therefore part of the burden of the law? Clearly, Reformed Covenant Theology accepts the New Testament canon containing the Pauline epistles and Acts as scripture, and so we can understand this view to be correct on the basis of authority. I would just like a better explanation than I have for why this sign has passed away or been swallowed up by Moses. For context, I am a Reformed Christian who affirms Reformed Covenant Theology.