“For the mouth speaks out of that which fills the heart.” (Matthew 12:34)
Jesus spoke these words to the Pharisees, who accused Him in :24 of casting out demons by the power of Beelzebub. He turned the focus back on them, saying that their hearts were not right with God, which is why they spoke against Jesus, and blasphemed the Holy Spirit. They spoke out of that which filled their hearts. And of course the same thing is true for people today. We don’t accidentally “slip” and say bad things. “The mouth speaks out of that which fills the heart.” Our words reveal what is inside us.
But this principle can also be true in a more positive way. When our hearts are full of good things, then our mouths will speak out of those good things which fill them. If the Holy Spirit, and His word, and the things of God, are filling our heart, then we will not be able to contain them: our mouths will speak out of that which fills our hearts!
Thus what many consider to be an American Christian “evangelism problem” — many Christians never bear witness about Christ to anyone — isn’t merely an “evangelism problem”; it is more than anything else a HEART problem. Our hearts are not filled with the Lord, or else we would be speaking of Him. Thus our challenge is not merely to “get out there and start sharing” by a sheer act of our will; rather our primary course of action should be to fill our hearts with Him — and when our hearts are full of the good things of God, then like the Apostles in Acts 4:20, “we cannot stop speaking about what we have seen and heard” — and which fills our hearts!