Our present object, however, is to expose the ignorance of Celsus, who makes this Jew of his address his fellow-citizen and the Israelitish converts in the following manner: “What induced you to abandon the law of your fathers?” etc. Now, how should they have abandoned the law of their fathers, who are in the habit of rebuking those who do not listen to its commands, saying, “Tell me, you who read the law, do you not hear the law? For it is written, that Abraham had two sons;” and so on, down to the place, “which things are an allegory,” etc.?
And how have they abandoned the law of their fathers, who are ever speaking of the usages of their fathers in such words as these: “Or does not the law say these things also? For it is written in the law of Moses, You shall not muzzle the mouth of the ox that treads out the grain. Does God care for oxen? Or says He it altogether for our sakes? For for our sakes it was written,” and so on? Now, how confused is the reasoning of the Jew in regard to these matters (although he had it in his power to speak with greater effect) when he says: “Certain among you have abandoned the usages of our fathers under a pretence of explanations and allegories; and some of you, although, as you pretend, interpreting them in a spiritual manner, nevertheless do observe the customs of our fathers; and some of you, without any such interpretation, are willing to accept Jesus as the subject of prophecy, and to keep the law of Moses according to the customs of the fathers, as having in the words the whole mind of the Spirit.”
Now how was Celsus able to see these things so clearly in this place, when in the subsequent parts of his work he makes mention of certain godless heresies altogether alien from the doctrine of Jesus, and even of others which leave the Creator out of account altogether, and does not appear to know that there are Israelites who are converts to Christianity, and who have not abandoned the law of their fathers? It was not his object to investigate everything here in the spirit of truth, and to accept whatever he might find to be useful; but he composed these statements in the spirit of an enemy, and with a desire to overthrow everything as soon as he heard it.
Source: Against Celsus (New Advent)