The Jew, then, continues his address to converts from his own nation thus: “Yesterday and the day before, when we visited with punishment the man who deluded you, you became apostates from the law of your fathers;” showing by such statements (as we have just demonstrated) anything but an exact knowledge of the truth. But what he advances afterwards seems to have some force, when he says: “How is it that you take the beginning of your system from our worship, and when you have made some progress you treat it with disrespect, although you have no other foundation to show for your doctrines than our law?”
Now, certainly the introduction to Christianity is through the Mosaic worship and the prophetic writings; and after the introduction, it is in the interpretation and explanation of these that progress takes place, while those who are introduced prosecute their investigations into “the mystery according to revelation, which was kept secret since the world began, but now is made manifest in the Scriptures of the prophets,” and by the appearance of our Lord Jesus Christ. But they who advance in the knowledge of Christianity do not, as you allege, treat the things written in the law with disrespect.
On the contrary, they bestow upon them greater honour, showing what a depth of wise and mysterious reasons is contained in these writings, which are not fully comprehended by the Jews, who treat them superficially, and as if they were in some degree even fabulous. And what absurdity should there be in our system— that is, the Gospel— having the law for its foundation, when even the Lord Jesus Himself said to those who would not believe upon Him: “If you had believed Moses, you would have believed Me, for he wrote of Me.
But if you do not believe his writings, how shall you believe My words?” Nay, even one of the evangelists— Mark— says: “The beginning of the Gospel of Jesus Christ, as it is written in the prophet Isaiah, Behold, I send My messenger before Your face, who shall prepare Your way before You,” which shows that the beginning of the Gospel is connected with the Jewish writings. What force, then, is there in the objection of the Jew of Celsus, that “if any one predicted to us that the Son of God was to visit mankind, he was one of our prophets, and the prophet of our God?”
Or how is it a charge against Christianity, that John, who baptized Jesus, was a Jew? For although He was a Jew, it does not follow that every believer, whether a convert from heathenism or from Judaism, must yield a literal obedience to the law of Moses.
Source: Against Celsus (New Advent)