One of the most powerful arguments employed by a number of the early Church Fathers for the divinity of Jesus Christ, the Son of God, was the doctrine of deification—i.e. if Jesus Christ was not God, he could not truly 'save' mankind through deification.
Perhaps the most famous example of this doctrine was from the pen of Athanasius:
For He was made man that we might be made God. (Athanasius - De Incarnation, 54; NPNF, second series, 4.65). [For more than 100 examples from the CFs, see THIS THREAD.]
Aquinas was a serious student of the Church Fathers, and retained the doctrine of deification in his thought. Note the following:
1459 The word "God" is also used in three senses. Sometimes it signifies the divine nature itself, and then it is used only in the singular: "Hear, O Israel: The Lord our God is one Lord" (Deut 6:4). At other times it is taken in a denominative sense: in this way idols are called gods: "All the gods of the peoples are idols" (Ps 96:5). And sometimes someone is called a god because of a certain participation in divinity, or in some sublime power divinely infused. In this way, even judges are called gods in Scripture: "If the thief is not known, the owner of the house shall be brought to the gods," that is, to the judges [Ex 22:8]; "You shall not speak ill of the gods," that is, of the rulers [Ex 22:28]. This is the way the word "god" is taken here, when he says, I said, you are gods, i.e., you share in some divine power superior to the human.
1460 Then when he says, If he called them gods to whom the word of God came, he shows the meaning of the authority he cited. This was like saying: He called them gods because they participated in something divine insofar as they participated in God's word, which was spoken to them. For due to God's word a person obtains some participation in the divine power and purity: "You are already made clean by the word which I have spoken to you" (15:3); and in Exodus (c 34) we read that the face of Moses shone when he heard the words of the Lord.
From what has been said above, one might argue in this way: It is clear that a person by participating in the word of God becomes god by participation. But a thing does not become this or that by participation unless it participates in what is this or that by its essence: for example, a thing does not become fire by participation unless it participates in what is fire by its essence. Therefore, one does not become god by participation unless he participates in what is God by essence. Therefore, the Word of God, that is the Son, by participation in whom we become gods, is God by essence. But our Lord, rather than argue so profoundly against the Jews, preferred to argue in a more human way. He says, and scripture cannot be broken, in order to show the irrefutable truth of Scripture: "O Lord, your word endures forever" [Ps 118:89]. (Commentary on the Gospel of John.)
Like so many of the Church Fathers before him, Aquinas argues that, "The Word of God, that is the Son", must be God, for if He is not God, then redeemed mankind could not become "Gods" (i.e. Sons of God).
More later, the Lord willing...
Grace and peace,