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You brood of vipers, how can you who are evil say anything good? For out of the overflow of the heart the mouth speaks. (Matt 12:34)

Matthew Henry Commentary:

Men's language discovers what country they are of, likewise what manner of spirit they are of. The heart is the fountain, words are the streams. A troubled fountain, and a corrupt spring, must send forth muddy and unpleasant streams. Nothing but the salt of grace, cast into the spring, will heal the waters, season the speech, and purify the corrupt communication. An evil man has an evil treasure in his heart, and out of it brings forth evil things. Lusts and corruptions, dwelling and reigning in the heart, are an evil treasure, out of which the sinner brings forth bad words and actions, to dishonour God, and hurt others. Let us keep constant watch over ourselves, that we may speak words agreeable to the Christian character.