Open Matthew 5–7.
John the Baptist was “the voice” announcing the arrival of God’s kingdom (3:1-3), introducing heaven’s anointed king (3:11-12). A voice from heaven confirmed his message: Jesus was indeed the chosen Son, the ruler heaven was pleased to appoint (3:17).
The anointed king had faced Israel’s enemy and driven him back: “Be gone, Satan: you have no authority here! Our Law honours YHWH our ruler. We serve no other” (4:10 paraphrased).
The king then withdrew to the northern reaches of his realm to live with the most oppressed, to bring light to the darkest place (4:12-16). There he announced the re-establishment of God’s kingdom, enacting the kingdom by releasing people from oppression by sickness and evil (4:22-25).
The king leads his followers to “a mountain” to give instruction on life in God’s kingdom. It was somewhere on the northern slopes of the Sea of Galilee, traditionally near Tabgha (Google maps). As you can see (photo above), it’s more of a hillock than a mountain. So why does Matthew call it a mountain? He’s thinking of something more than geography.
When Israel became God’s kingdom, where did Moses lead them? Mount Sinai was where they entered the covenant to have YHWH as their sovereign and to be his people (Exodus 19:5-6). Moses went up the mountain to receive the revelation of their sovereign, i.e. his instructions for how they were to live under his rule.
But the kingdom that represented God among the nations — the kingdom founded at Mount Sinai — had fallen apart generations ago (Matthew 1:17). It needed to be re-established. That’s why Matthew uses Sinai imagery to describe the instruction they received from their new king. Moses “went up the mountain” (Deuteronomy 9:9), taking the elders of Israel (Exodus 24:9), while the crowd waited below. Surrounded by crowds hoping for the restoration of the kingdom, Jesus “went up the mountain” to give kingdom instruction to the disciples he had chosen (5:1).
As Matthew unfolds his account of the Good News, he will call this a new covenant, a new Passover, the kingdom under the one to whom all authority is given. In that sense, the Sermon on the Mount is a fresh Torah — the instruction that establishes the relationship between the king and his people.
The king requires his people to care for each other in the same way they care for themselves. He begins by acknowledging those trapped in the current injustices, announcing that they will be restored to blessing under his rule (5:1-12). He gives purpose to his citizens (5:13-16), promising to fulfil what heaven commands (5:17-20). He explains how they are to obey their heavenly sovereign’s commands — so important, since they represent on earth his perfect character (5:21-48).
He explains how to live in relation to their heavenly sovereign (6:1-18), requiring them to seek his kingship above all else (6:19-34). He legislates how they are to live in relation to each other: it all comes down to loving unselfishly (7:1-12). It won’t do to simply call him ruler: they must be productive citizens (7:13-23). They stand or fall depending on their obedience to their king’s commands (7:24-29).
Little wonder our king’s instruction (the Sermon on the Mount) has inspired people from the earliest Christian writings (The Didache) to world leaders such as Mahatma Ghandi. It’s the foundation document of the kingdom of God. The king’s brother was right to call this “the royal law” (James 2:8).
What others are saying
Craig G. Bartholomew and Michael W. Goheen, The Drama of Scripture: Finding Our Place in the Biblical Story. (Grand Rapids, MI: Baker Academic, 2004) 143:
The Sermon on the Mount is our clearest example of Jesus’ instruction to his disciples about life in the kingdom of God.
Craig S. Keener, The Gospel of Matthew: A Socio-Rhetorical Commentary (Grand Rapids, MI: Eerdmans, 2009), 164:
Matthew chose to emphasize the presence of a mountain, and probably not for matters of purely topographic interest. Many scholars think that Matthew here recalls Moses’ revelation on Mount Sinai (Ex 19:3; 24:13, 18). If so, Jesus’ superior revelation also makes him superior to those who “sit in Moses’ seat” (23:2; Patte 1987: 61–62, 68); Jewish teachers advised one to receive a Torah scholar’s words with fear and trembling as if one received them from Sinai (ARN 6 A). The one greater than Moses (cf. Is 2:3; 33:22), first encountered in 2:13–20, has begun his mission.
Brevard S. Childs, Biblical Theology of the Old and New Testaments: Theological Reflection on the Christian Bible (Minneapolis, MN: Fortress Press, 2011), 541–542:
In the Sermon on the Mount, the Evangelist offers the fullest account of Jesus’ radicalization of the law, which was not simply a correction of rabbinic interpretation, but of the Old Testament law itself. In the antitheses between the biblical tradition (‘You have heard it said …’) and Jesus’ new rendering of the will of God, Jesus placed himself above Moses as God’s true interpreter. Jesus saw himself fully in line with the Old Testament prophets (12:7; 15:8) when he interpreted the heart of the law as loving God and neighbour (22:34ff.). …
Matthew’s understanding of the law stands in closest continuity with the Old Testament in its demands for life before God in radical obedience to his will which is reflected in the fruits of righteousness. … However, the major break with the Old Testament tradition lies in Matthew’s christological witness which focusses the entire discussion of the law on Jesus, who as God’s long awaited Anointed One, and sovereign interpreter of the law, confronts Israel with the radical nature of God’s will through word and deed.