When weak becomes strong

Two of the most common "adversative" or "negative" conjunctions in Greek are δέ (de), and ἀλλὰ (alla). δέ is the weaker of the two, and can be translated either "and" or "but" depending on the context. ἀλλὰ, on the other hand, is a stronger contrast. Of the 628 times it appears in the New Testament, only once is it translated "and," and in this instance, it still carries an adversative idea (Mt. 18:30). The vast majority of the time, it is translated "but" or "yet."

In the Sermon on the Mount, when Jesus contrasts the external rules of the Jewish tradition with the internal laws of His Messianic kingdom, I naturally expected Jesus to choose the strong adversative ἀλλὰ. But this isn't the case! In all six instances, He actually chooses the weaker conjunction δέ:

Matthew 5:22 But I tell you,     ἐγὼ δὲ λέγω ὑμῖν

Matthew 5:28 But I tell you,     ἐγὼ δὲ λέγω ὑμῖν

Matthew 5:32 But I tell you,     ἐγὼ δὲ λέγω ὑμῖν

Matthew 5:34 But I tell you,     ἐγὼ δὲ λέγω ὑμῖν

Matthew 5:39 But I tell you,     ἐγὼ δὲ λέγω ὑμῖν

Matthew 5:44 But I tell you,     ἐγὼ δὲ λέγω ὑμῖν

Why didn’t Jesus use the stronger ἀλλὰ to show the sharp contrast between the Jewish traditions and His own teaching? Perhaps it's because contrast has already been supplied by the pronoun ἐγὼ, which is emphatic. Lit., He says, “But I Myself tell you.” Leon Morris writes,

“[Jesus] uses the emphatic ἐγώ. France comments, “This is not a new contribution to exegetical debate, but a definitive declaration of the will of God. It demands (and receives, 7:28–29) the response, ‘Who is this?’ Thus this passage contributes another aspect to the presentation of Jesus as the Messiah which is Matthew’s overriding purpose.”

I suspect that in this case, using the stronger adversative conjunction ἀλλὰ may have actually stolen emphasis away from the pronoun, where Jesus wanted all His shock and emphasis to land. So Jesus instead chose a weaker conjunction and put all the emphasis squarely on the pronoun ἐγὼ. He says, "It is I alone, and not the tradition of the elders, who will instruct you in God's Law."

A new authority had arrived in town. His name was Jesus, and He was singlehandedly overturning centuries of oral tradition with one sermon.