Wow… Today I read this from John MacArthur’s commentary
When our Lord told the Pharisees that “the mouth speaks out of that which fills the heart” (Matt. 12:34), He gave an important spiritual principle:
Speech will reflect the kind of person one is. Because the tongue can speak so easily and is difficult to control, a person’s speech becomes the truest indicator of his spiritual state (cf. Matt. 12:37).
The Bible has much to say about the speech of both the redeemed and the unredeemed mouth.
The unredeemed mouth is characterized by evil (Prov. 15:28), sexual immorality (Prov. 5:3), deceit (Jer. 9:8), curses (Ps. 10:7), oppression (Ps. 10:7), lies (Prov. 12:22), destruction (Prov. 11:11), vanity (2 Pet. 2:18), flattery (Prov. 26:28), foolishness (Prov. 15:2), madness (Eccles. 10:12–13), carelessness (Matt. 12:36), boasting (Rom. 1:30), false doctrine (Titus 1:11), evil plots (Ps. 37:12), hatred (Ps. 109:3), too many words (Eccles. 10:14), and gossip (Prov. 26:22).
In contrast, redeemed speech is characterized by confession of sin (1 John 1:9), confession of Christ (Rom. 10:9–10), edifying speech (Eph. 4:29), talk of God’s law (Ex. 13:9), praise to God (Heb. 13:15), blessing of enemies (1 Pet. 3:9), talk about God (Ps. 66:16), wisdom and kindness (Prov. 31:26), and gentleness (Prov. 15:1). It takes as its model the Lord Jesus, who spoke instructively (Matt. 5:2), graciously (Luke 4:22), blamelessly (Luke 11:54), and without deceit (1 Pet. 2:22).