20 But you, whoever you are that would seek into the unsearchable, and in all seriousness form an opinion upon the mysteries and powers of God—I turn to you for counsel, and beg you to enlighten me, an unskilled and simple believer of all that God says, as to a circumstance which I am about to mention. I listen to the Lord's words and, since I believe what is recorded, I am sure that after His Resurrection He offered Himself repeatedly in the Body to the sight of multitudes of unbelievers.
At any rate, He did so to Thomas who had protested that he would not believe unless he handled His wounds. His words are, Unless I shall see in His hands the print of the nails, and put my finger into the place of the nails, and thrust my hand into His side, I will not believe. The Lord stoops to the level even of our feeble understanding; to satisfy the doubts of unbelieving minds He works a miracle of His invisible power. Do you, my critic of the ways of heaven, explain His action if you can.
The disciples were in a closed room; they had met and held their assembly in secret since the Passion of the Lord. The Lord presents Himself to strengthen the faith of Thomas by meeting his challenge; He gives him His Body to feel, His wounds to handle. He, indeed, who would be recognised as having suffered wounds must needs produce the body in which those wounds were received. I ask at what point in the walls of that closed house the Lord bodily entered. The Apostle has recorded the circumstances with careful precision; Jesus came when the doors were shut, and stood in the midst. Did He penetrate through bricks and mortar, or through stout woodwork, substances whose very nature it is to bar progress?
For there He stood in bodily presence; there was no suspicion of deceit. Let the eye of your mind follow His path as He enters; let your intellectual vision accompany Him as He passes into that closed dwelling. There is no breach in the walls, no door has been unbarred; yet lo, He stands in the midst Whose might no barrier can resist. You are a critic of things invisible; I ask you to explain a visible event. Everything remains firm as it was; no body is capable of insinuating itself through the interstices of wood and stone.
The Body of the Lord does not disperse itself, to come together again after a disappearance; yet whence comes He Who is standing in the midst? Your senses and your words are powerless to account for it; the fact is certain, but it lies beyond the region of human explanation. If, as you say, our account of the Divine birth is a lie, then prove that this account of the Lord's entrance is a fiction. If we assume that an event did not happen, because we cannot discover how it was done, we make the limits of our understanding into the limits of reality.
But the certainty of the evidence proves the falsehood of our contradiction. The Lord stood in a closed house in the midst of the disciples; the Son was born of the Father. Deny not that He stood, because your puny wits cannot ascertain how He came there; renounce a disbelief in God the Only-begotten and perfect Son of God the Unbegotten and perfect Father, which is based only on the incapacity of sense and speech to comprehend the transcendent miracle of that birth.
Source: On the Trinity (New Advent)