2 Corinthians 3:18
“But we all, with unveiled face, reflecting as a mirror the glory of the Lord.”
Not that which is brought to an end, but that which remains.
“Are transformed into the same image from glory to glory, even as from the Lord the Spirit.”
Do you see how again he places the Spirit in the rank of God, (vide infra) and raises them up to the rank of the Apostles. For he said before, “You are the Epistle of Christ;” and here, “But we all with open face.” Yet they came, like Moses, bringing a law. But like as we, he says, needed no veil, so neither ye who received it. And yet, this glory is far greater, for this is not of our countenance, but of the Spirit; but nevertheless you are able as well as we to look steadfastly upon it. For they indeed could not even by a mediator, but you even without a mediator can [look steadfastly on] a greater. They were not able to look upon that of Moses, you even upon that of the Spirit. Now had the Spirit been at all inferior, He would not have set down these things as greater than those. But what is, “we reflecting as a mirror the glory of the Lord, are transformed into the same image.” This indeed was shown more clearly when the gifts of miracles were in operation; howbeit it is not even now difficult to see it, for one who has believing eyes. For as soon as we are baptized, the soul beams even more than the sun, being cleansed by the Spirit; and not only do we behold the glory of God, but from it also receive a sort of splendor. Just as if pure silver be turned towards the sun's rays, it will itself also shoot forth rays, not from its own natural property merely but also from the solar lustre; so also does the soul being cleansed and made brighter than silver, receive a ray from the glory of the Spirit, and send it back. Wherefore also he says, “Reflecting as a mirror we are transformed into the same image from glory,” that of the Spirit, “to glory,” our own, that which is generated in us; and that, of such sort, as one might expect from the Lord the Spirit. See how here also he calls the Spirit, Lord. And in other places too one may see that lordship of His. For, says he, “As they ministered and fasted unto the Lord, the Spirit said, Separate me Paul and Barnabas.” For therefore he said, “as they ministered unto the Lord, Separate me,” in order to show the [Spirit's] equality in honor. And again Christ says, “The servant knows not what his lord does;” but even as a man knows his own things, so does the Spirit know the things of God; not by being taught [them,] for so the similitude holds not good. Also the working as He wills shows His authority and lordship. This transforms us. This suffers not to be conformed to this world; for such is the creation of which This is the Author. For as he says, “Created in Christ Jesus,” so says he, “Create in me a clean heart, O God, and renew a right spirit in my inward parts.”
Source: Homilies on Second Corinthians (New Advent)