Charles Spurgeon Commentary Matthew 12:34-35

"Ye offspring of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh. The good man out of his good treasure bringeth forth good things: and the evil man out of his evil treasure bringeth forth evil things." — Matthew 12:34-35 (ASV)

Our Lord accuses them of being evil. He repeats John’s words, O generation of vipers! They had spoken evil. How could they do otherwise when their hearts were so full of malice towards Him? They had gone to the utmost extreme of malevolence in charging Him with being in league with Satan, and that only showed what a treasure of evil lay within their hearts.

They threw evil forth with energy of temper and with lavishness of falsehood, because they had such a fullness of it within. That which is in the well comes up in the bucket. The heart betrays itself through the mouth. Had they been good, their words would have been good, but such was their baseness of heart that they could not speak good things. Thus our Lord carried the war into their own territory and flashed holy indignation in their faces.