MATTHEW'S INTRODUCTION
“SEEING the crowds, He went up on the mountain; and when He was seated, His disciples came to Him. Then He opened His mouth and taught them, saying.” (Matthew 5:1-2 Amplified Version)
Matthew's introductory sentence goes a long way. In those brief verses there are three clues to the real significance of the Sermon on the Mount.
(i) Jesus began to teach when he had sat down. When a Jewish Rabbi was teaching officially he sat to teach. We still speak of a professor's chair; the Pope still speaks ex cathedra, from his seat. Often a Rabbi gave instruction when he was standing or strolling about; but his official teaching was done when he had taken his seat. So the very suggestion that Jesus sat down to teach the crowd is the indication that this teaching is central and official.
(ii) Matthew goes on to say that when he had opened his mouth, he taught them. This phrase he opened his mouth is not simply a decoratively roundabout way of saying he said. In Greek the phrase has a double significance. (a) It is used of a solemn, grave and dignified utterance. It is used, for instance, of the saying of an oracle. It is the natural preface for a most weighty saying. (b) It is used of a person's utterance when he is really opening his heart and fully pouring out his mind. It was used of intimate teaching with no barriers between. Again, the very use of this phrase indicates that the material in the Sermon on the Mount is no chance piece of teaching. It is the grave and solemn utterance of the central things; it is the opening of Jesus' heart and mind to everyone.
(iii) When Jesus had sat down, he opened his mouth and taught them saying. In Greek there are two past tenses of the verb. There is the aorist tense, and the aorist tense expresses one particular action, done and completed in past time. In the sentence, " He shut the gate," shut would be an aorist in Greek because it describes one completed action in past time. There is the imperfect tense, and the imperfect tense describes repeated, continuous, or habitual action in past time. In the sentence, " It was his custom to go to Church every Sunday," would be expressed by a single verb in the imperfect tense, because it describes continuous and often-repeated action in the past.
Now the point is that in the Greek of this sentence, the verb taught is not an aorist, but an imperfect and therefore it describes repeated and habitual action, and the translation should be: "This is what he used to teach them." Matthew has said as plainly as Greek will say it that the Sermon on the Mount is not one sermon of Jesus, given at one particular time and on one particular occasion; it is the essence of all that Jesus continuously and habitually taught his disciples and everyone.
The Sermon on the Mount is greater even than we think. Matthew in his introduction wishes us to see that it is the official teaching of Jesus; that it is the opening of Jesus' whole mind to his disciples and everyone; that it is the summary of the teaching, which Jesus habitually gave to his inner circle. The Sermon on the Mount is nothing less than the concentrated memory of many hours of heart to heart communion between the disciples and their Master. In other words, a heart to heart communion between us and our Master.