5 He showed himself then to the disciples. What is “Himself”? The Head of His Church. The Church was foreseen by Him as in you to be throughout the world, by the disciples it was not yet seen. He showed the Head, He promised the Body. For what did He add next? “These are the words which I spoke to you, while I was yet with you.” What is this, “While I was yet with you”? Was He not with them then when He was speaking to them? What is, “when I was yet with you”? Was with you as mortal, which now I am not.
I was with you when I had yet to die. What is, “with you”? With you who were to die, Myself to die. Now I am no more with you: for I am with those who are to die, Myself to die no more for ever. This then is what I said to you. What? “That all things must be fulfilled which are written in the Law, and in the Prophets, and in the Psalms concerning Me.” I told you that all things must be fulfilled. “Then opened He their understanding.” Come then, O Lord, employ Your keys, open, that we may understand.
Lo, Thou dost tell all things, and yet are not believed. You are thought to be a spirit, art touched, art rudely handled, and yet they who touch You hesitate. Thou dost admonish them out of the Scriptures, and yet they understand You not. Their hearts are closed, open, and enter in. He did so. “Then opened He their understanding.” Open, O Lord, yea, open the heart of him who is in doubt concerning Christ. Open “his” understanding who believes that Christ was a phantom. “Then opened He their understanding, that they might understand the Scriptures.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)