Matthew 12:34

O generation of vipers
Though they boasted of their being the seed of Abraham, yet their immediate ancestors were no other than vipers, deceitful, hurtful, poisonous creatures; and they were exactly like them: for though they made a fair show in the flesh, and outwardly appeared righteous, yet were inwardly full of the poison of wickedness, envy and malice; and which their pestilential breath, their blasphemy against the Spirit, fully discovered; and gave just cause and reason for so severe a reproof, and such resentment, as here made by Christ.

How can ye, being evil, speak good things?
This is not to be expected, nor is it commonly and constantly done; an evil man may sometimes speak good things, or which seem to be so; but these are not his common talk; as he is, so, for the most part, is his language; his speech betrays him: and since these men were by nature evil, were destitute of the Spirit and grace of God, had no good thing in them, how should any good thing come out of them? And since they were so full of wickedness, spite and malice, it is no wonder that they belched out such blasphemous expressions concerning the miracles of Christ;

for out of the abundance of the heart the mouth speaketh:
a phrase much like this is used by the Septuagint, in ( Ecclesiastes 2:15 ) . "I spoke abundance", or "much in my heart"; (dioto o afrwn ek perisseumatov) (lalei) , "for the fool out of his abundance speaketh": as there is abundance of folly in him, there is much delivered out by him; and where there is abundance of wickedness in the heart, if the grace of God is wanting to restrain it, much of it will come out by the lips; as is a man's heart, ordinarily is his language.