9 If we are asked why we do not worship God as the Hebrew fathers of the Old Testament worshipped Him, we reply that God has taught us differently by the New Testament fathers, and yet in no opposition to the Old Testament, but as that Testament itself predicted. For it is thus foretold by the prophet: "Behold, the days come, says the Lord, when I will make a new covenant with the house of Israel, and with the house of Judah; not according to the covenant which I made with their fathers when I took them by the hand to bring them out of the land of Egypt." Thus it was foretold that that covenant would not continue, but that there would be a new one.
And to the objection that we do not belong to the house of Israel or to the house of Judah, we answer according to the teaching of the apostle, who calls Christ the seed of Abraham, and says to us, as belonging to Christ's body, "Therefore you are Abraham's seed." Again, if we are asked why we regard that Testament as authoritative when we do not observe its ordinances, we find the answer to this also in the apostolic writings; for the apostle says, "Let no man judge you in meat or drink, or in respect of a holiday, or a new moon, or of Sabbaths, which are a shadow of things to come." Here we learn both that we ought to read of these observances, and acknowledge them to be of divine institution, in order to preserve the memory of the prophecy, for they were shadows of things to come; and also that we need pay no regard to those who would judge us for not continuing the outward observance; as the apostle says elsewhere to the same purpose, "These things happened to them for an example; and they are written for our admonition, on whom the end of the ages have come." So, when we read anything in the books of the Old Testament which we are not required to observe in the New Testament, or which is even forbidden, instead of finding fault with it, we should ask what it means; for the very discontinuance of the observance proves it to be, not condemned, but fulfilled. On this head we have already spoken repeatedly.
Source: Reply to Faustus the Manichaean (New Advent)