Theophylact of Ohrid

And He opened His mouth. Why does the evangelist say, "He opened His mouth"? It would appear that this is superfluous; but it is not. For He also taught without opening His mouth. How did He do this? By His life and His miracles. But now He opens His mouth and teaches. And taught them, saying. He taught not only His disciples, but the multitude as well. He begins with the beatitudes, "Blessed are they," just as David began the Psalms with the beatitude, "Blessed is the man" (Ps. 1:1).

Commentary on Matthew ↗

Augustine of Hippo

Or, the phrase is introductory of an address longer than ordinary. (ubi sup.) Whoever will take the trouble to examine with a pious and sober spirit, will find in this sermon a perfect code of the Christian life as far as relates to the conduct of daily life. Accordingly the Lord concludes it with the words, Every man who heareth these words of mine and doeth them, I will liken him to a wise man, & c.

Catena Aurea by Aquinas ↗

Gregory the Dialogist

When the Lord on the mountain is about to utter His sublime precepts, it is said, Opening his month he taught them, He who had before opened the mouth of the Prophets.

Catena Aurea by Aquinas ↗

Thomas Aquinas

He opened his mouth and taught them. By saying, he opened, is signified that for a long time previously he had been silent. And it shows that he was about to make a great and long sermon, as Augustine says. Or that he was about to utter great and profound things; for men were wont to do that: "After this, Job opened his mouth and cursed the day of his birth" (Jb 3:1). …

Commentary on Matthew ↗