And he opened his mouth and taught them, saying:
2. Opening his mouth. This redundancy of expression (πλεονασμὸς) partakes of the Hebrew idiom: for what would be faulty in other languages is frequent among the Hebrews, to say, He opened his mouth, instead of, He began to speak. Many look upon it as an emphatic mode of expression, employed to draw attention to any thing important and remarkable, either in a good or bad...
We have here a general account of this sermon. I. The Preacher was our Lord Jesus, the Prince of preachers, the great Prophet of his church, who came into the world, to be the Light of the world. The prophets and John had done virtuously in preaching, but Christ excelled them all.
Commenting on Matthew 5:1-2
And he opened his mouth,.... He spoke with a clear and strong voice, that all the people might hear him; and with great freedom, utterance, and cheerfulness, and things of the greatest moment and importance; and taught them; not his disciples only, but the whole multitude, who heard him with astonishment; see Mat 7:28.