After this he continues: “Their union is the more wonderful, the more it can be shown to be based on no substantial reason. And yet rebellion is a substantial reason, as well as the advantages which accrue from it, and the fear of external enemies. Such are the causes which give stability to their faith.” To this we answer, that our union does thus rest upon a reason, or rather not upon a reason, but upon the divine working, so that its commencement was God's teaching men, in the prophetical writings, to expect the advent of Christ, who was to be the Saviour of mankind.
For in so far as this point is not really refuted (although it may seem to be by unbelievers), in the same proportion is the doctrine commended as the doctrine of God, and Jesus shown to be the Son of God both before and after His incarnation. I maintain, moreover, that even after His incarnation, He is always found by those who possess the acutest spiritual vision to be most God-like, and to have really come down to us from God, and to have derived His origin or subsequent development not from human wisdom, but from the manifestation of God within Him, who by His manifold wisdom and miracles established Judaism first, and Christianity afterwards; and the assertion that rebellion, and the advantages attending it, were the originating causes of a doctrine which has converted and improved so many men was effectually refuted.
Source: Against Celsus (New Advent)