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...
Christ the eternal Wisdom, who lay in the Father's bosom before all worlds, preached in a mountain because the scribes had Moses' chair with all ease and honor while corrupting the law; He found no better pulpit than a desert hill. His miracles made way for this sermon, disposing people to receive instruction from one showing such divine power and goodness.
AI summary
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.