BECAUSE IF THEY ARE NOT:
J.C. Ryle talking about why Jesus was circumcised on the eighth day:
Without [circumcision] [Jesus] would not have fulfilled the law’s requirements. Without it He could not have been recognized as the son of David, and the seed of Abraham. Let us remember, furthermore, that circumcision was absolutely necessary before our Lord could be heard as a teacher in Israel. Without it He would have had no place in any lawful Jewish assembly, and no right to any Jewish ordinance. Without it He would have been regarded by all Jews as nothing better than an uncircumcised Gentile, and an apostate from the faith of the fathers.
J. C. Ryle, Expository Thoughts on Luke Volume I, Banner of Truth Trust, 1986, p. 62.
Michael Horton on the continuity from Old Testament circumcision to New Testament children baptism:
3. Children were included in the covenant of grace in the Old Testament through the sacrament of circumcision, and in the new covenant (called the “better covenant”), God has not changed in his good intentions toward our children (Acts 2:38). Circumcision has been replaced by baptism (Col. 2:11). Therefore, our children must receive God’s sign and seal of covenant ownership.
4. The children of unbelievers are unholy, but the children of believers are set apart unto God. This is a distinction not only of the Old Testament (see the Passover, Exod. 12:42-51; also the distinction between the “house of the wicked” and the “house of the righteous,” especially in the psalms) but is continued in the New, where a believer’s children are regarded as holy (1 Cor. 10:2). How are they marked or distinguished from unbelievers, then? By the sign and seal of the covenant.
M. S. Horton, A Better Way, Baker Books, 2002, p. 106-107.