“And seeing the multitudes, he went up into a mountain: and when he was set, his disciples came unto him:”
Seeing the crowds, he went up on the mountain, and when he sat down, his disciples came to him. The setting of the Sermon on the Mount is theologically charged: the mountain recalls Sinai, where Moses received the law, and Jesus ascending the mountain and sitting to teach (the posture of authoritative teaching in Jewish tradition) signals that what follows is a new authoritative proclamation. But Matthew has been careful: the crowds are present, not only the disciples, and Jesus' sitting draws his disciples to him specifically. The Sermon is addressed to disciples in the hearing of the crowds — it describes the life of those who have already begun to follow, not the conditions for entry. Hebrews 12:18–24 contrasts the fearful mountain of the law with the joyful assembly of the new covenant; this mountain is where the new covenant's demands and gifts are proclaimed.
Church Fathers on Matthew 5:1
Every man in his own trade or profession rejoices when he sees an opportunity of exercising it; the carpenter if he sees a goodly tree desires to have it to cut down to employ his skill on, and the Priest when he sees a full Church, his heart rejoices, he is glad of the occasion to teach. So the Lord seeing a great con greg ation of people was stirred to teach them.
Or He may be thought to have sought to shun the thickest crowd, and to have ascended the mountain that He might speak to His disciples alone.
By not choosing His seat in the city, and the market place, but on a mountain in a desert, He has taught us to do nothing with ostentation, and to depart from crowds, above all when we are to be employed in philosophy, or in speaking of serious things.