Matthew 12:34

“O generation of vipers, etc.] 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…”

“O generation of vipers--(See on Mat 3:7). how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh--a principle obvious enough, yet of deepest significance and vast application. In Luk 6:45 we find it uttered as part of the discourse delivered after the choice of the apostles.”

“O generation of vipers! - Christ here applies the argument which he had suggested in the previous verse. They were a wicked race; like poisonous reptiles, with a corrupt and evil nature. They could not be expected to speak good things - that is, to speak favorably of him and his works. As the bad fruit of a tree was the proper effect of its “nature,” so were their words about him and his works the proper effect of their nature. The “abundance” or fullness of the “heart” produced the words of the lips. “Vipers” are a poisonous kind of serpents, not often a yard long, and about an inch thick, having a flat head. The males have two large teeth, through which a most deadly poison is thrown into…”

“In another kind likewise, the tree is known by its fruit - Namely, the heart by the conversation. Matthew 12:36”

“generation: Mat 3:7, Mat 23:33; Luk 3:7; Joh 8:44; Jo1 3:10 how: Sa1 24:13; Psa 10:6, Psa 10:7, Psa 52:2-5, Psa 53:1, Psa 64:3, Psa 64:5, Psa 120:2-4, Psa 140:2, Psa 140:3; Isa 32:6, Isa 59:4, Isa 59:14; Jer 7:2-5; Rom 3:10-14; Jam 3:5-8 for: Luk 6:45 Matthew 12:35”