If Celsus had read the Scriptures in an impartial spirit, he would not have said that “our writings are incapable of admitting an allegorical meaning.” For from the prophetic Scriptures, in which historical events are recorded (not from the historical), it is possible to be convinced that the historical portions also were written with an allegorical purpose, and were most skilfully adapted not only to the multitude of the simpler believers, but also to the few who are able or willing to investigate matters in an intelligent spirit.
If, indeed, those writers at the present day who are deemed by Celsus the “more modest of the Jews and Christians” were the (first) allegorical interpreters of our Scriptures, he would have the appearance, perhaps, of making a plausible allegation. But since the very fathers and authors of the doctrines themselves give them an allegorical signification, what other inference can be drawn than that they were composed so as to be allegorically understood in their chief signification? And we shall adduce a few instances out of very many to show that Celsus brings an empty charge against the Scriptures, when he says “that they are incapable of admitting an allegorical meaning.”
Paul, the apostle of Jesus, says: “It is written in the law, You shall not muzzle the mouth of the ox that treads out the grain. Does God take care for oxen? Or says He it altogether for our sakes? For our sakes, no doubt, this is written, that he that ploughs should plough in hope, and he that threshes in hope of partaking.” And in another passage the same Paul says: “For it is written, For this cause shall a man leave his father and mother and shall be joined to his wife, and they two shall be one flesh.
This is a great mystery; but I speak concerning Christ and the Church.” And again, in another place: “We know that all our fathers were under the cloud, and all passed through the sea; and were all baptized unto Moses in the cloud, and in the sea.” Then, explaining the history relating to the manna, and that referring to the miraculous issue of the water from the rock, he continues as follows: “And they did all eat the same spiritual meat, and did all drink the same spiritual drink.
For they drank of that spiritual Rock that followed them, and that Rock was Christ.” Asaph, moreover, who, in showing the histories in Exodus and Numbers to be full of difficulties and parables, begins in the following manner, as recorded in the book of Psalms, where he is about to make mention of these things: “Give ear, O my people, to my law: incline your ears to the words of my mouth. I will open my mouth in parables; I will utter dark sayings of old, which we have heard and known, and our fathers have told us.”
Source: Against Celsus (New Advent)