Out of the overflow 

What is in a man's heart will always make itself known in both his words, and his actions, "for the mouth speaks out of that which fills the heart" (Matt. 12:34b). Although all humans are sometimes embarrassed by what they say and would deny that their words mean anything, man's words actually reveal what is in his heart. The Bible says this throughout. As King Saul sought to kill David, David would not lift his hand against Saul, even when given the opportunity to do so.

David told Saul, "As the proverb of the ancients says, 'Out of the wicked comes forth wickedness;' but my hand shall not be against you" (1 Sam. 24:13). Isaiah 32:6 says, "For a fool speaks nonsense, and his heart inclines toward wickedness: to practice ungodliness and to speak error against the Lord, to keep the hungry person unsatisfied and to withhold drink from the thirsty."

In the NT, Matthew 15:18 says, "But the things that proceed out of the mouth come from the heart, and those defile the man." Paul wrote, "Let no unwholesome word proceed from your mouth, but only such a word as is good for edification according to the need of the moment, so that it will give grace to those who hear" (Eph. 4:29; cf. Jas. 3:2-12).

A person's mouth acts like an overflow line on a bathtub or air conditioner unit. What is in man's heart will always overflow through his mouth. He will speak of his lusts, his anger, his covetousness, his self-righteousness, his selfish desires, his hatreds—everything that fills his heart and all of his natural inclinations. This is standard talk for all who do not know Jesus as the Christ, but sadly, it is also true of Christians. Christians are saved and are thus aware of how their old nature sometimes overwhelms their new nature in Christ, but they are far more prone, as a new creature in Christ, to use their mouths as an overflow of love, joy, and peace. After all, these fruits of the Spirit (Gal. 5:22-23) are the evidence of their new nature in Christ. Yet, as stated, sometimes the old nature peeks through, sadly but surely.

Verse 35 speaks of the "good man," one who "brings out of his good treasure what is good." This is just another way of saying that he is a man declared righteous through faith in Jesus Christ who is like the good tree that naturally bears good fruit.

The term for "treasure" comes from the Greek term thēsauros which is used in English for "repository; storehouse." The storehouse of the "good man" is a heart full of the fruit of the Spirit (Gal. 5:22-23), and his mouth and his actions overflow with that fruit, bringing forth what is good. The evil man, on the other hand, does the exact opposite, bringing forth rotten fruit from both his actions and his mouth. This exposes his heart as "evil," for only an evil heart can bear such rotten fruit.
 
Whereas the words of scoffers who hate Christ and the words of hypocrites who feign a love for Christ are almost always rotten fruit, the words of the godly are a blessing.
 
"The words of the godly are like sterling silver; the heart of a fool is worthless. The words of the godly encourage many, but fools are destroyed by their lack of common sense" (Proverbs 10:20-21, NLT).

"Timely advice is lovely, like golden apples in a silver basket. To one who listens, valid criticism is like a gold earring or other gold jewellery" (Proverbs 25:11-12, NLT).

"Let your conversation be gracious and attractive so that you will have the right response for everyone" (Colossians 4:6, NLT).