9 Accordingly, when you ask why a Christian is not circumcised if Christ came not to destroy the law, but to fulfill it, my reply is, that a Christian is not circumcised precisely for this reason, that what was prefigured by circumcision is fulfilled in Christ. Circumcision was the type of the removal of our fleshly nature, which was fulfilled in the resurrection of Christ, and which the sacrament of baptism teaches us to look forward to in our own resurrection. The sacrament of the new life is not wholly discontinued, for our resurrection from the dead is still to come; but this sacrament has been improved by the substitution of baptism for circumcision, because now a pattern of the eternal life which is to come is afforded us in the resurrection of Christ, whereas formerly there was nothing of the kind.
So, when you ask why a Christian does not keep the Sabbath, if Christ came not to destroy the law, but to fulfill it, my reply is, that a Christian does not keep the Sabbath precisely because what was prefigured in the Sabbath is fulfilled in Christ. For we have our Sabbath in Him who said, "Come unto me, all you that labor and are heavy laden, and I will give you rest. Take my yoke upon you, and learn of me; for I am meek and lowly in heart, and you shall find rest unto your souls."
Source: Reply to Faustus the Manichaean (New Advent)