"The good man brings good things out of the good stored up in his heart, and the evil man brings evil things out of the evil stored up in his heart. For out of the overflow of his heart his mouth speaks" (Luke 6:45).
"The Lord says: 'These people come near to me with their mouth and honor me with their lips, but their hearts are far from me. Their worship of me is made up only of rules taught by men'” (Isaiah 29:13).
People have God-given escape valves to keep them from exploding. Laughter and tears are two of them. The mouth is another. The cure for a bad mouth is to keep good stored up in your heart.
However, not everything that comes out of the mouth is from the heart. Some of what comes out of people’s mouths is just what they have been taught, whether true or false, like in Isaiah 29:13. This is why people can recite sound doctrine but lose their temper at home.
The real person is the heart person, not the head person. We are two persons—the person taught by men and the person of the heart. Can these two be the same? Yes, but it is not automatic. We can teach children that it is wrong to lie. That is not the same as teaching children not to lie. The first is teaching the head; the second is teaching the heart.
Not everything that comes out of the mouth comes from the heart. Do the good truths you speak come from your heart, or only from your head?