By his unexpected entry through closed doors Christ proved once more that by nature he was God and also that he was none other than the one who had lived among them. By showing his wounded side and the marks of the nails, he convinced us beyond a doubt that he had raised the temple of his body, the very body that had hung upon the cross. He restored that body which he had worn, destroying death’s power over all flesh, for as God, he was life itself. Why would he need to show them his hands and side if, as some perversely think, he did not rise again bodily. And if the goal was not to have the disciples think about him in this way, why not appear in another form and, disdaining any likeness of the flesh, conjure up other thoughts in their minds? But he obviously thought it was that important to convince them of the resurrection of his body that, even when events would have seemed to call for him to change the mode of his body into some more ineffable and surpassing majesty, he nonetheless resolved in his providence to appear once more as he had been in the past [i.e. in the flesh] so that they might think he was wearing any other form than the one in which he had suffered crucifixion. Our eyes couldn’t have endure the glory of his holy body, if he had chosen to reveal it to his disciples before he ascended to the Father. Anyone who reflects on the transfiguration will easily infer this is the case. . . . since, it says, they could not endure the sight but fell on their faces.
Commentary on the Gospel of St. John 12.1.