4 Forasmuch then as we could in no way have had this blessedness by which we see not and yet believe, unless we received it of the Holy Ghost; it is with good reason said, “It is expedient for you that I go away. For if I go not away, the Comforter will not come unto you; but if I depart, I will send Him unto you.” By His Divinity indeed He is with us always; but unless He had in Body gone away from us, we had always seen His Body after the flesh. and never believed after a spiritual sort; by the which belief justified and blessed we might attain with cleansed hearts to contemplate the Very Word, God with God, “by whom all things were made,” and “who was made Flesh, that He might dwell among us.”
And if not with the contact of the hand, but “with the heart man believes unto righteousness;” with good reason is the world, which will not believe save what it sees, convinced of our righteousness. Now that we might have that righteousness of faith of which the unbelieving world should be convinced, therefore said the Lord, “Of righteousness, because I go to the Father, and you shall see Me no more.” As if He had said, This shall be your righteousness, that you believe in Me, the Mediator, of whom you shall be most fully assured that He is risen again and gone to the Father, though you see Him not after the Flesh; that by Him reconciled, you may be able to see God after the Spirit.
Whence He says to the woman who represents the Church, when she fell at His Feet after His Resurrection, “Touch Me not, for I am not yet ascended to the Father.” Which expression is understood mystically, thus. “Believe not in Me after a carnal manner by means of bodily contact; but you shall believe after a spiritual manner; that is, with a spiritual faith shall touch Me, when I shall have ascended to the Father.” For, “blessed are they who do not see, and believe.” And this is the righteousness of faith, of which the world, which has it not, is convinced of us who are not without it; for “the just lives by faith.” Whether it be then that as rising again in Him, and in Him coming to the Father, we are invisibly and in justification perfected; or that as not seeing and yet believing we live by faith, for that “the just lives by faith.” with these meanings said He, “Of righteousness, because I go to the Father, and you shall see Me no more.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)