Together with Matthew 7: 28 – 8: 1, Matthew 5: 1-2 forms the narrative frame of the ‘Sermon on the Mount.’ The introductory verses in Matthew 5: 1-2 present a solemn setting for the Sermon. Jesus ascends the mountain and assumes a magisterial position. He then begins his authoritative teaching. The disciples and the crowds (cf. Mt 5: 1; 7: 28-29) form the audience for the Sermon.
The expression “seeing the crowds” in Matthew 5: 1 relates closely the Sermon on the Mount to the preceding passage which mentions that “great crowds followed him…” (cf. Mt 4: 25). Jesus seems to address his message directly to his disciples (Mt 5: 1b-2) but the crowds too were there listening to him and at the end of the Sermon the crowds react to Jesus’ teaching with astonishment (Mt 7: 28). By presenting the crowds as part of Jesus’ audience the evangelist wants to stress the point that the message of the Sermon on the Mount is intended for all and not merely for the inner circle of Jesus’ disciples. The location of the Sermon is “the mountain.” It is not named, and it is futile to seek to identify this mountain. “The mountain” in Matthew’s gospel is not so much a geographical location as it is a place for divine revelation and other important actions (cf. Mt 4: 8-10; 17: 1-8; 28: 16-20; 14: 23; 15: 29). For Matthew then, unlike gospel of Luke 6: 17, the mountain is a fitting place for Jesus’ Sermon in which he reveals the new law of the Kingdom and the “great charter of discipleship.”
The setting of the Sermon on “the mountain” also enhances its importance. Here there may also be an allusion to MOUNT SINAI and the giving of the Law in the Old Testament. The evangelist also mentions that Jesus “sat down.” Sitting was the position adopted by the Jewish rabbis/teachers and it was sign of their authority and dignity as teachers. So then at the beginning of the beginning of the Sermon on the Mount Matthew presents Jesus as an authoritative teacher and that the teaching that follows is something important and official. Again, the additional phrase “he opened his mouth” is also intended to increase the solemnity of this significant teaching. In this sense the phrase “he opened his mouth” is found elsewhere in the Bible (cf. Ps 78: 2; Acts 8: 35; 10: 34 etc). Thus in Matthew 5: 1-2 the evangelist sets the stage and solemnly introduces Jesus’ teaching on the “Sermon on the Mount.”