33 “Either make the tree good and its fruit good, or else make the tree bad and its fruit bad; for a tree is known by its fruit.
34 Brood of vipers! How can you, being evil, speak good things? For out of the abundance of the heart the mouth speaks.
35 A good man out of the good treasure of his heart brings forth good things, and an evil man out of the evil treasure brings forth evil things.
36 But I say to you that for every idle word men may speak, they will give account of it in the day of judgment.
37 For by your words you will be justified, and by your words you will be condemned.”
Say that you know the tree is corrupt because its fruit is corrupt; or dare to say that the tree is good, because its fruit is good. There is in these words the touch of a great pity, of a great desire to help these men. He appealed to them not to attribute good fruit to a corrupt tree. That is what they were doing. They said that the good result, of a man freed from demon possession, was wrought by Satan. He appealed to them to be consistent; to believe on Him for the very works’ sake.
Then finally He uttered the most awe-inspiring words of all; “Ye offspring of vipers, how can ye, being evil, speak good things?” The severity of that consists in His evident pity for them. It seems as if even the hopeful spirit of Christ was almost hopeless about these men. He said, How can ye speak a true thing? How can ye say the tree is good because its fruit is good? How can you tell the truth? You are the offspring of vipers; you are morally degenerate; you have lost your moral discrimination. — Morgan, page 132.
In short, Christ was pointing out the evilness of their accusation that His good miracles were done in the power of Satan.
In contrast, since they acted in the power of Satan, nothing they produce could possibly be good.
The idle words the Lord referred to were their words about Him. Their words indicated that they rejected Christ and attributed His power to Satan. For those words, they would be judged. From this point on, Christ’s words and actions demonstrate that He considered the nation of Israel to have rejected Him.