Offspring of vipers. The similarity between the tree and the fruit is here applied by Christ to nothing more than speech, because this afforded an opportunity of detecting the inward and concealed malice of the scribes; and that is the reason why he dwells so much on this one kind of sin. It is because their falsehood and slanders betrayed what was not so visible in the rest of their life, that Christ attacks them with such severity. "There is no reason to wonder," he says, "that you vomit out wicked words; for your heart is full of malice." Nor are we to suppose that he ought to have treated them with greater gentleness, because some might regard this reproof as excessively severe. There are other sins, no doubt, that call for harsh reproofs; but when hypocritical persons pervert what is right, or put a false coloring on what is sinful, such wickedness renders it necessary that God should thunder against it in a more terrible manner than against other sins. Now the design of Christ, suggested by the present occurrence, was to condemn the wicked sophistry which turns light into darkness. This passage shows how highly valuable in the sight of the Lord truth is, since he maintains and defends it with such rigor. Would that this were earnestly considered by those persons, whose ingenuity is too ready to be employed in defending any cause, and whose venal tongue disguises impostures! In a particular manner, Christ waxes wroth against those whom ambition, or envy, or some other fraudulent design, prompts to slander, even when there is nothing that their conscience condemns. Against the Pharisees, too, as his custom was, Christ used greater harshness, because they were so captivated by an unfounded conviction of their righteousness, that an ordinary warning had no effect upon them. And till hypocrites are sharply pierced, [1] all that is said to them is treated with scorn and contempt. How can you speak what is good? We have formerly hinted, that proverbial sayings ought not to be rigidly interpreted as an invariable rule, for they state nothing more than the ordinary fact. Sometimes, no doubt, a cruel man will deceive the simple by honied flatteries, a cunning man will cheat under the garb of simplicity, and a man of very wicked thoughts will breathe almost angelical purity of language. [2] But the ordinary practice demonstrates the truth of what Christ here says, that out of the abundance of the heart the mouth speaketh; agreeably to the old proverb, which declared the tongue to be the index of the mind. [3] And, indeed, whatever hidden and crooked recesses may exist in the heart of man, and whatever may be the amazing contrivances by which every man conceals his vices, yet the Lord extorts from each of them some kind of confession, so that they discover by the tongue their natural disposition and hidden feelings. We must also observe the purpose for which Christ employs those parables; for he reproaches the Pharisees with having manifested by words the malice which they had inwardly conceived. Besides, knowing them to be sworn enemies, he takes occasion from, single calumny to expose their whole life, and to destroy their credit with the people, which gave them too great influence in deceiving and in doing mischief. Though good speeches do not always proceed from the inmost heart, but originate (as the phrase is) on the tip of the tongue, yet it is an invariable truth, that bad speeches are indications of a bad heart.

Footnotes

1 - "Iusques a ce qu'ils sentent qu'on le poigne a bon escient;" -- "till they feel that they are pierced in good earnest."

2 - "Il semblera, a l'ouir parler, qu'il ait une purete Angelique;" -- "to hear him speak, you would imagine that he has the purity of an Angel."

3 - "Comme aussi le proverbe ancien portoit, Que la langue est le charactere ou pourtrait du coeur;" -- "as also the old proverb bore, That the tongue is the type or portrait of the heart."

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 the wound made by the bite. They are an emblem of malignity and mischief. These were strong expressions to be used by the meek and lowly Jesus; but they were not the effect of anger and malice; they were a declaration of the true character of the people with whom he was conversing - a declaration most justly deserved. See the notes at Matthew 3:7.

O generation of vipers - These are apparently severe words; but they were extremely proper in reference to that execrable people to whom they were addressed: the whole verse is an inference from what was spoken before.
Out of the abundance (περισσευματος, the overflowings) of the heart - Wicked words and sinful actions may be considered as the overflowings of a heart that is more than full of the spirit of wickedness; and holy words and righteous deeds may be considered as the overflowings of a heart that is filled with the Holy Spirit, and running over with love to God and man.

(7) O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh.
(7) Hypocrites at length betray themselves even by their own mouth.

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"; "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.

O generation of vipers--(See on Matthew 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 Luke 6:45 we find it uttered as part of the discourse delivered after the choice of the apostles.

Generation of vipers. Poisonous, evil and dangerous, like vipers.
How can ye, etc.? Their hearts were thoroughly evil, hence they could only speak and do evil, for "out of the good treasure of the heart the mouth speaketh." On the other hand, the "good man out of the good treasure of his heart bringeth forth good things."

In another kind likewise, the tree is known by its fruit - Namely, the heart by the conversation.