Thomas Aquinas, whose feastday was recently added to the Episcopal Church sanctoral, is stereotypically regarded as a dry, quintessentially cerebral theologian, with nary a mystical bone in his body.  That this is a gross oversimplification is shown by a number of passages in his works referring to theosis or deification, which we stereotypically pigeonhole as an Eastern Christian concept.  This aspect of his thought is addressed by Daniel A Keating in his Deification and Grace (Sapientia Press 2007). 

A hat tip to the site Joe Versus the Volcano.

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Commentary on Ephesians (3:20)
The human mind and will  could never imagine, understand or  ask that God become  man, and that man become God and a sharer in the divine nature.  But he has done this in us by his power, and it was accomplished in the Incarnation of his Son.

Commentary on John (15:9)
The Son did not love the disciples in either of these ways.  For he did not love them to the point of their being gods by nature, nor to the point that they would be united to God so as to form one person with him.  But he did love them up to a similar point:  he loved them to the extent that they would be gods by their participation in grace--"I say, 'You are gods'" (Ps 82:6).

Summa Theologiae (I-II, q. 112, a. 1)
Nothing can act beyond its species, since the cause must always be more powerful than its effect.  Now the gift of grace surpasses every capability of created nature, since it is nothing short of a partaking of the divine nature, which exceeds every other nature.  And thus it is impossible that any creature should cause grace.  For it is as necessary that God alone should deify, bestowing a partaking of the divine nature by a participated likeness as it is impossible that anything save fire should enkindle.