5 I give unceasing thanks to my teacher, who prevented me from falling into this error, so that I am still a Christian. For I, like you, from reading this verse without sufficient consideration, had almost resolved to become a Jew. And with reason; for if Christ came not to destroy the law, but to fulfill it, and as a vessel in order to be filled full must not be empty, but partly filled already, I concluded that no one could become a Christian but an Israelite, nearly filled already with the law and the prophets, and coming to Christ to be filled to the full extent of his capacity.
I concluded, too, that in thus coming he must not destroy what he already possesses; otherwise it would be a case, not of fulfilling, but of emptying. Then it appeared that I, as a Gentile, could get nothing by coming to Christ, for I brought nothing that he could fill up by his additions. This preparatory supply is found, on inquiry, to consist of Sabbaths, circumcision, sacrifices, new moons, baptisms, feasts of unleavened bread, distinctions of foods, drink, and clothes, and other things, too many to specify.
This, then, it appeared, was what Christ came not to destroy, but to fulfill. Naturally it must appear so: for what is a law without precepts, or prophets without predictions? Besides, there is that terrible curse pronounced upon those who abide not in all things that are written in the book of the law to do them. With the fear of this curse appearing to come from God on the one side, and with Christ on the other side, seeming, as the Son of God, to say that he came not to destroy these things, but to fulfill them, what was to prevent me from becoming a Jew? The wise instruction of Manichæus saved me from this danger.
Source: Reply to Faustus the Manichaean (New Advent)