Renewal through the Holy Spirit consists, first of all, of the grace that cleanses. Sin is a sort of old age of the soul, and a man is only freed from this old age through justifying grace, by which he is cleansed from sin…: “As Christ has risen from the dead, so also let us walk in newness of life” (Rom. 6:4).
[...] Secondly, this renewal consists in the justice that is ever making progress. If one should walk, grow tired, and become weak, and then he rests, his powers seem to him to be renewed; and when a man works diligently, he is renewed when he gains further power for working.
[...] Thirdly, renewal comes about through the wisdom that illuminates. When a man comes to new knowledge of more of the good things of God, he is renewed. About this renewal it says in Colossians: “Put on the new man who is created according to God.”
The “new man” indicates Christ, because His was a new kind of conception, “not from the seed of man, but from the Holy Spirit”;
a new kind of birth, because His mother remained a virgin after birth; a new kind of suffering, because it was without guilt; a new kind of rising from the dead, because it was quick and renewing, for He rose quickly and in glory;
a new kind of ascension, because he ascended by His own power, not by that of another, as did Enoch and Elijah. And so it is said in Ecclesiasticus: “Show signs anew and work wonders” (Sir. 36:6).
And because all things are renewed through Christ, therefore on solemnities we use new vestments in church, that we may “sing to the Lord a new song”—as though to signify that he who is renewed by the exterior cleanness of his clothing is renewed interiorly in his mind by grace.
By “stripping off the old man,” i.e., the habit of sins with its deeds, “and putting on” the habit of virtue which is not lacking in [good] deeds, “the new man,” i.e., the rational mind, will be renewed “in the knowledge of God” (Col. 3:9-10). As Romans has it, “Put on the Lord Jesus Christ” (Rom. 13:14).
[...] Fourthly, renewal comes about through the glory that attains consummation, when the body is renewed, the oldness of punishment and guilt being taken away. We read about this in the prophet Isaiah: “Behold, I create new heavens and a new earth;” (Is. 65:17).
And where does this renewal come from? The Holy Spirit. He is the pledge of our inheritance, and it is He who leads us into the heavenly inheritance. He who needs to be created and renewed shall obtain this from the Holy Spirit.
Thomas Aquinas (1225-1274): Sermon “Emitte Spiritum”.