St Augustine on the Sermon on the Mount.

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The beginning, then, of this sermon is introduced as follows: And when He saw the great multitudes, He went up into a mountain: and when He was set, His disciples came unto Him: and He opened His mouth, and taught them, saying. If it is asked what the mountain means, it may well be understood as meaning the greater precepts of righteousness; for there were lesser ones which were given to the Jews. Yet it is one God who, through His holy prophets and servants, according to a thoroughly arranged distribution of times, gave the lesser precepts to a people who as yet required to be bound by fear; and who, through His Son, gave the greater ones to a people whom it had now become suitable to set free by love. Moreover, when the lesser are given to the lesser, and the greater to the greater, they are given by Him who alone knows how to present to the human race the medicine suited to the occasion. Nor is it surprising that the greater precepts are given for the kingdom of heaven, and the lesser for an earthly kingdom, by that one and the same God, who made heaven and earth. With respect, therefore, to that righteousness which is the greater, it is said through the prophetYour righteousness is like the mountains of God: and this may well mean that the one Master alone fit to teach matters of so great importance teaches on a mountain. Then He teaches sitting, as behooves the dignity of the instructor’s office; and His disciples come to Him, in order that they might be nearer in body for hearing His words, as they also approached in spirit to fulfil His precepts. And He opened His mouth, and taught them, saying. The circumlocution before us, which runs, And He opened His mouth, perhaps gracefully intimates by the mere pause that the sermon will be somewhat longer than usual, unless, perchance, it should not be without meaning, that now He is said to have opened His own mouth, whereas under the old law He was accustomed to open the mouths of the prophets.