Matthew 12:34

Then since He is arguing not for Himself, but for the Spirit, He hath dealt out His reproof even as a torrent, saying, "O generation of vipers, how can ye, being evil, speak good things?" Now this is at once to accuse, and to give demonstration of His own sayings from their case. For behold, saith He, ye being evil trees, cannot bring forth good fruit. I do not then marvel at your talking thus: for ye were both ill nurtured, being of wicked ancestors, and ye have acquired a bad mind. And see how carefully, and without any hold for exception, He hath expressed His accusations: in that He said not, "How can ye speak good things, being a generation of vipers? (for this latter is nothing to the former): but, "How can ye, being evil, speak good things?" But He called them "broods of vipers," because they prided themselves on their forefathers. To signify therefore that they had no advantage thereby, He both casts them out from their relationship to Abraham, and assigns them forefathers of kindred disposition, having stripped them of that ground of illustriousness. "For out of the abundance of the heart the mouth speaketh." Here again He indicates His Godhead, which knew their secrets: and that not for words only, but also for wicked thoughts, they shall suffer punishment; and that He knows it all, as God. And He saith, that it is possible even for men to know these things; for this is a natural consequence, that when wickedness is overflowing within, its words should be poured forth through the lips. So that when thou hearest a man speak wicked words, do not suppose only so much wickedness to be in him as the words display, but conjecture the fountain to be much more abundant; for that which is spoken outwardly, is the superabundance of that which is within. See how vehemently He reprehends them. For if what they had said is so evil, and is of the very mind of the devil, consider the root and well-spring of their words, how far that must reach. And this is naturally the case; for while the tongue through shame often pours not forth all its wickedness at once, the heart having no human witness, fearlessly gives birth to whatever evils it will; for of God it hath not much regard. Since then men's sayings come to examination: and are set before all, but the heart is concealed; therefore the evils of the former grow less, while those of the latter increase. But when that within is multiplied, all that hath been awhile hidden comes forth with a violent gushing. And as persons vomiting strive at first to keep down the humors that force their way out, but, when they are overcome, cast forth much abomination; so do they that devise evil things, and speak ill of their neighbors. And what can be more barbarous than this, to overlook our soul that is more precious than all, abused by so many adulterers, and so long companying with them, even until they are sated; which will never be.

Or the words, Generation of vipers, may be taken as signifying children, or imitators of the Devil, because they had wilfully spoken against good works, which is of the Devil, and thence follows, Out of the abundance of the heart the mouth speaketh. That man speaks out of the abundance of the heart who is not ignorant with what intention his words are uttered; and to declare his meaning more openly He adds, A good man out of the good treasure of his heart bringeth forth good things. The treasure of the heart is the intention of the thoughts, by which the Judge judges that work which is produced, so that sometimes though the outward work that is shown seem great, yet because of the carelessness of a cold heart, they receive a little reward from the Lord.