18 But know this, that all those things which were seen in bodily form were not that substance of God. For we saw those things with the eyes of the flesh: how is the substance of God seen? Interrogate the Gospel: “Blessed are the pure in heart; for they shall see God.” There have been men who, deceived by the vanity of their hearts, have said, The Father is invisible, but the Son is visible. How visible? If on account of His flesh, because He took flesh, the matter is manifest.
For of those who saw the flesh of Christ, some believed, some crucified; and those who believed doubted when He was crucified; and unless they had touched the flesh after the resurrection, their faith would not have been recalled. If, then, on account of His flesh the Son was visible, that we also grant, and it is the Catholic faith; but if before He took flesh, as they say, that is, before He became incarnate, they are greatly deluded, and grievously err. For those visible and bodily appearances took place though the creature, in which a type might be exhibited: not in any fashion was the substance itself shown and made manifest.
Give heed, beloved brethren, to this easy proof. The wisdom of God cannot be beheld by the eyes. Brethren, if Christ is the Wisdom of God and the Power of God; if Christ is the Word of God, and if the word of man is not seen with the eyes, can the Word of God be so seen?
Source: Tractates on the Gospel of John (New Advent)