8 But to the perils of tempests are added also the errors of heretics; and there are not wanting those who so try the minds of them that are in the ship, as to say that Christ was not born of a Virgin, nor had a real body, but seemed to the eyes what He was not. And these opinions of heretics have sprung up now, when the Name of Christ is already glorified throughout all nations; when Christ, that is, is as it were now walking on the sea. The disciples in their trial said, “It is a phantom.” But He gives us strength against these pestilent opinions by His own voice, “Be of good cheer, it is I; be not afraid.” For men in vain fear have conceived these opinions concerning Christ, looking at his Honour and Majesty; and they think that He could not be so born, who has deserved to be so Glorified, fearing Him as it were “walking on the sea.”
For by this action the excellency of His honour is figured; and so they think that He was a phantom. But when he says, “It is I;” what else does He say but that there is nothing in Him which does not really exist? Accordingly if He shows His flesh, it is flesh; if bones, they are bones; if scars, they are scars. For “there was not in Him yea and nay, but in Him was yea,” as the Apostle says. Hence that expression, “Be of good cheer, it is I; be not afraid.” That is, do not so stand in awe of My Majesty, as to wish to take away the reality of My Being from Me.
Though I walk upon the sea, though I have under My feet the elation and the pride of this world, as the raging waves, yet have I appeared as very Man, yet does My Gospel proclaim the very truth concerning Me, that I was born of a Virgin, that I am the Word made flesh; that I said truly, “Handle Me, and see, for a spirit has not bones as you see Me have,” that they were true impresses of My wounds which the hands of the doubting Apostle handled. And therefore “It is I; be not afraid.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)