The topic of the sermon is that God works wonders in His saints, and St. Thomas treats of this topic by taking a verse from the Psalms about David:
I have discovered David my servant; with my holy oil I have anointed him;
my hand will help him, and my arm will strengthen him (Ps. 88:21-22).
This gives us a series of wonders that God works in the servants of God -- David in particular, of course, but also any servant of God. And thus Thomas uses it to speak of how St. Nicholas was such a servant. There are four basic parts to the verse, to which Aquinas assigns one feature of God's wondrous working in the saints:
(1) I have discovered David my servant: election
(2) with my holy oil I have anointed him: consecration
(3) my hand will help him: execution of duties
(4) and my arm with strengthen him: steadfastness
Thus Thomas will show wondrous election, singular consecration, effective execution of office, and abiding steadfastness in St. Nicholas. Actually, he never gets to the last; the sermon we have stops abruptly and without explanation after (3).
I have discovered David my servant, the Psalmist says; what's involved in discovering someone? Discovery, says Thomas, suggests rarity, at least to the extent that it needs to be discovered; it suggests search; it suggests disclosure; and it suggests conviction through experience. All these are elements of God's wonderful choosing of St. Nicholas: the first in that St. Nicholas was virtuous from youth, the second in that the Lord seeks faithful souls to delight in; the third in that Nicholas stood out through his pious affection and profound mercy and compassion; and the fourth in that Nicholas faithfully served the Lord's interests rather than his own. The third is particularly important for Aquinas; St. Nicholas is an example he holds up in more than one place for his compassion and mercy. He clearly likes the story of St. Nicholas finding a way to give gold in secret to the poor virgins so that they could have a dowry without the embarrassment of being beholden to him for it. Notably Thomas also uses his discussion of Nicholas's election to attack abuses by the clergy.
According to legend, St. Nicholas was elevated to the position of bishop by God Himself. The old bishop had died with no one obvious as a replacement. Those who were trusted with choosing the successor had a dream one night that they should consecrate as bishop the first man who walked through the door of the Church that morning. This happened to be Nicholas, who was at the time a young priest and a newcomer to Myra. He took considerable convincing, but eventually he was installed as bishop. This is perhaps subtly in the background here, although Aquinas doesn't mention it explicitly here (he does explicitly mention it elsewhere, so he knew of the story). Instead he focuses on the phrase with my holy oil I have anointed him. Oil has four uses, says Aquinas, all of which are suggested in this context.
First, oil is used for healing. Thus oil is an image of God's healing grace, and we see the operation of such grace in such a holy man as Nicholas.
Second, oil is used for lighting. To this extent it symbolizes the learning of wisdom, which is why it is associated with prophecy and illumination.
Third, oil is used for flavoring. In this sense it is an image of spiritual joy; just as a sprinkling of oil makes food taste better, so does a sprinkling of spiritual joy make good works easier. It is in this sense that oil is associated with priesthood.
Fourth, oil is used for softening and smoothing. Understood in this way it signifies mercy and kindness of heart which, of course, St. Nicholas had in astounding measure. Thus, says Thomas Aquinas, just as oil spreads itself out, so does mercy, and just as oil coats things, so mercy coats every good work. He then has a very interesting passage:
You ought to consider that in the future, according to the merits of graces the evidence of rewards will appear in the glorified bodies of the saints, and that even in this life the signs of their affection appear. This is evident in the case of blessed Francis, where the signs of the passion of Christ became visible, so vehemently was he affected by the passion of Christ. In blessed Nicholas's case, signs of mercy appeared when "his tomb sweated oil," thus indicating that he was a man of great mercy.
The linking of the two extremely popular saints, Saint Francis and Saint Nicholas, is rather interesting in itself, since, while Nicholas founded no order, there are nonetheless a great many similarities between the two, as regards their place in the Church and what they have left for posterity. It has also not gone without notice that here Thomas the Dominican takes explicit trouble to mention the stigmata of Saint Francis, which has suggested to some that his audience may have been Franciscan.
In any case, Thomas holds that this fourth signification of oil is why oil is often associated with kingship.
And thus in these four ways, divine grace, prophetic wisdom, priestly gladness, and kingly compassion, God works wonders in His saints.
My hand shall help him. The hand symbolizes God's strength, and Thomas suggests four ways in which God's strength is found to operate in saints like Nicholas. First, God drew Nicholas to Himself and away from evil. Second, God guided Nicholas as He does all the just. Third, He gave him strength and comfort. And fourth, because Nicholas showed exquisite mercy, God worked miracles through him.
And this is how the sermon ends, abruptly but memorably:
It was mercy that made blessed Nicholas an extraordinary man, and the Lord strengthened him even unto everlasting life. May He lead us there, who lives with the Father and the Holy Spirit, &c.