10 When you ask why a Christian does not observe the distinction in food as enjoined in the law, if Christ came not to destroy the law, but to fulfill it, I reply, that a Christian does not observe this distinction precisely because what was thus prefigured is now fulfilled in Christ, who admits into His body, which in His saints He has predestined to eternal life, nothing which in human conduct corresponds to the characteristics of the forbidden animals. When you ask, again, why a Christian does not offer sacrifices to God of the flesh and blood of slain animals, if Christ came not to destroy the law, but to fulfill it, I reply, that it would be improper for a Christian to offer such sacrifices, now that what was thus prefigured has been fulfilled in Christ's offering of His own body and blood.
When you ask why a Christian does not keep the feast of unleavened bread as the Jews did, if Christ came not to destroy the law, but to fulfill it, I reply, that a Christian does not keep this feast precisely because what was thus prefigured is fulfilled in Christ, who leads us to a new life by purging out the leaven of the old life. When you ask why a Christian does not keep the feast of the paschal lamb, if Christ came not to destroy the law, but to fulfill it, my reply is, that he does not keep it precisely because what was thus prefigured has been fulfilled in the sufferings of Christ, the Lamb without spot.
When you ask why a Christian does not keep the feasts of the new moon appointed in the law, if Christ came not to destroy the law, but to fulfill it, I reply, that he does not keep them precisely because what was thus prefigured is fulfilled in Christ. For the feast of the new moon prefigured the new creature, of which the apostle says: "If therefore there is any new creature in Christ Jesus, the old things have passed away; behold, all things have become new." When you ask why a Christian does not observe the baptisms for various kinds of uncleanness according to the law, if Christ came not to destroy the law, but to fulfill it, I reply, that he does not observe them precisely because they were figures of things to come, which Christ has fulfilled.
For He came to bury us with Himself by baptism into death, that as Christ rose again from the dead, so we also should walk in newness of life. When you ask why Christians do not keep the feast of tabernacles, if the law is not destroyed, but fulfilled by Christ, I reply that believers are God's tabernacle, in whom, as they are united and built together in love, God condescends to dwell, so that Christians do not keep this feast precisely because what was thus prefigured is now fulfilled by Christ in His Church.
Source: Reply to Faustus the Manichaean (New Advent)