And so it was, when Jesus had ended these sayings, that the people were astonished at His teaching, for He taught them as one having authority, and not as the scribes. (Matthew 7:28-29, NKJV).
The last verse of the Sermon on the Mount ends, appropriately, with the people's reaction. Christ's listeners were astonished at His teaching. Why? The answer is given in the next phrase, for He taught them as one having authority, and not as the scribes.
Jesus was unequivocal in His statements. He straddled no fences, and presented no gray areas. He did not augment His instructions with precedent citations, or bolster His arguments with the wisdom, writings or opinions of others. He presented Himself as the single authoritative source. Such a stance was astonishing to His audience precisely because He was speaking as if He were the author of these cosmic and eternal truths. There were only two conclusions possible. He was either a blaspheming heretic, wickedly saying things like, “You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ “But I say to you, love your enemies…" (Matthew 5:43, 44, NKJV). Or He was God. C.S. Lewis summarizes it best:
I am trying here to prevent anyone saying the really foolish thing that people often say about Him: 'I'm ready to accept Jesus as a great moral teacher, but I don't accept His claim to be God.' That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic -- on the level with the man who says he is a poached egg -- or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon; or you can fall at His feet and call Him Lord and God. But let us not come with any patronizing nonsense about His being a great human teacher. He has not left that open to us. He did not intend to. – Mere Christianity, pages 40-41.
And that is another reason for their astonishment. Christ was forcing them, and us, to make a choice. He was not trying to sell them something, or lure them into anything against their will. In fact, He was doing the diametric opposite: He was challenging them to willfully accept His authority. It was a bold and unprecedented move. The people were used to the scribes' manner of teaching, putting forth this or that interpretation of some law, and then presenting those who agreed, disagreed, or supplemented that view, much like our modern judicial decisions. The scribe presenting himself as the final arbiter of meaning was very bad form. Jesus would have none of that prevarication. He was the final and only arbiter of meaning, and He made no bones about it.
I can imagine that throughout the entire discourse, some reacted with dismay, or disagreement, or indignation at His words. Others may have listened with joy at what He was revealing in clear and unmistakable terms. They may have understood that a veil of misunderstanding was being lifted and were amazed that they were granted the privilege of seeing first light. Yet overall, the crowd was astonished. Open-mouthed in wonder and shock. They were, literally, struck, as if by a physical blow.
Jesus drawing attention to Himself this way was immeasurable mercy on His part. Think about it. The Law had been given clearly by Moses and the Prophets. God's expectations and standards had been painstakingly revealed to the world through the Jews, who were chosen to receive "the oracles of God". He had demonstrated unmistakably in history who He was, and what He required. With perfect justice, He could have then condemned the world in its stubborn enmity, and obliterated it. That Jesus came and further revealed these things, not only by words, but by example, was immeasurably gracious, an indescribable act of unmerited favor. That He remained to pay the price of our sin by dying on the Cross is that and much more; beyond human comprehension.
Yes, when we consider who He was and what He condescended to do on our behalf, we too should be astonished, struck speechless in gratitude and love. We have nothing to give Him in return but ourselves, and the only value in that is what He has imputed to us by His love and grace.
I encourage you, study these chapters in Matthew. Meditate upon them. Hide them in your heart and mind. God's Word does not return to Him void. If done with honesty, diligence, and sincere desire, it can not help but change your life, just as Jesus intended.