{1} And he opened his mouth, and taught them, saying,

(1) Christ teaches that the greatest joy and happiness is not in the conveniences and pleasures of this life, but is laid up in heaven for those who willingly rest in the good will and pleasure of God, and endeavour to profit all men, although they are cruelly vexed and troubled by those of the world, because they will not adapt themselves to their ways.

People's New Testament

5:2 Opened his mouth, and taught. This wonderful discourse of three chapters is to the New Dispensation what the law given from Sinai was to the Old. That was the moral law of Judaism, this is the moral law of Christianity; that was given from the Mount that could not be touched, this from the Mount of blessing. Compare Lu 6:20-49.

Wesley's Notes

5:2 And he opened his mouth - A phrase which always denotes a set and solemn discourse; and taught them - To bless men; to make men happy, was the great business for which our Lord came into the world. And accordingly he here pronounces eight blessings together, annexing them to so many steps in Christianity. Knowing that happiness is our common aim, and that an innate instinct continually urges us to the pursuit of it, he in the kindest manner applies to that instinct, and directs it to its proper object. Though all men desire, yet few attain, happiness, because they seek it where it is not to be found. Our Lord therefore begins his Divine institution, which is the complete art of happiness, by laying down before all that have ears to hear, the true and only true method of acquiring it. Observe the benevolent condescension of our Lord. He seems, as it were, to lay aside his supreme authority as our legislator, that he may the better act the part of: our friend and Saviour. Instead of using the lofty style, in positive commands, he, in a more gentle and engaging way, insinuates his will and our duty, by pronouncing those happy who comply with it.

Scofield Reference Notes

[2] And he opened his mouth

Having announced the kingdom of heaven as "at hand," the King, in Mat 5.-7., declares the principles of the kingdom. The Sermon on the Mount has a twofold application:

(1) literally to the kingdom. In this sense it gives the divine constitution for the righteous government of the earth. Whenever the kingdom of heaven is established on earth it will be according to that constitution, which may be regarded as an explanation of the word "righteousness" as used by the prophets in describing the kingdom (e.g.) Isa 11:4,5 32:1 Dan 9:24 In this sense the Sermon on the Mount is pure law, and transfers the offence from the overt act to the motive. Mt 5:21,22,27,28. Here lies the deeper reason why the Jews rejected the kingdom. They had reduced "righteousness" to mere ceremonialism, and the Old Testament idea of the kingdom to a mere affair of outward splendour and power. They were never rebuked for expecting a visible and powerful kingdom, but the words of the prophets should have prepared them to expect also that only the poor in spirit and the meek could share in it (e.g.) Isa 11:4. The seventy-second Psalm, which was universally received by them as a description of the kingdom, was full of this. For these reasons, the Sermon on the Mount in its primary application gives neither the privilege nor the duty of the Church. These are found in the Epistles. Under the law of the kingdom, for example, no one may hope for forgiveness who has not first forgiven. Mt 6:12,14,15. Under grace the Christian is exhorted to forgive because he is already forgiven. Eph 4:30-32.

(2) But there is a beautiful moral application to the Christian. It always remains true that the poor in spirit, rather than the proud, are blessed, and those who mourn because of their sins, and who are meek in the consciousness of them, will hunger and thirst after righteousness, and hungering, will be filled. The merciful are "blessed," the pure in heart do "see God." These principles fundamentally reappear in the teaching of the Epistles.

Margin saying

The beatific character, unattainable by effort, is wrought in the believer by the Spirit, Gal 5:22,23.

Jamieson-Fausset-Brown Bible Commentary

2. And he opened his mouth-a solemn way of arousing the reader's attention, and preparing him for something weighty. (Job 9:1; Ac 8:35; 10:34).

and taught them, saying-as follows.

Matthew Henry's Concise Commentary

5:1,2 None will find happiness in this world or the next, who do not seek it from Christ by the rule of his word. He taught them what was the evil they should abhor, and what the good they should seek and abound in.