Matthew 12:35

“The good man out of his good treasure brings forth good things, and the evil man out of his evil treasure brings forth evil things.”

So a good man will speak what is in his heart and will produce good things. That is because his heart is filled with good treasure. The idea of treasure is that it has been stored up by choice (Matthew 6:19). And good treasure stands the test. It is pure and righteous and compassionate and merciful. It is filled with love (Matthew 5:44). In the words of Paul it ‘suffers long and is kind, it does not envy, or exalt itself, or be puffed up, it does not behave itself in an unseemly way, it is not easily provoked. It does not rejoice in iniquity but rejoices in the truth. It bears all things, believes all things, hopes all things, endures all things. It never fails' (1 Corinthians 13:4). Thus it will bring forth good things, and in context, good words from the mouth. And this will be true both in public and in private. But the man who is evil has an evil treasure within, a treasure that is false, and that also is what will issue from his mouth.