Luke 6:45
The good man out of the good treasure of his heart brings out that which is good, and the evil man out of the evil treasure of his heart brings out that which is evil, for out of the abundance of the heart, his mouth speaks.
WEB
A good man out of the good treasure of his heart bringeth forth that which is good; and an evil man out of the evil treasure of his heart bringeth forth that which is evil: for of the abundance of the heart his mouth speaketh.
KJV
What Luke 6:45 means
Jesus says that words are overflow, that whatever a person stores up in the heart eventually spills out of the mouth, which makes speech a surprisingly honest readout of what someone truly loves.
This line closes a short teaching of Jesus, in Luke's Sermon on the Plain, about trees and their fruit. A good tree bears good fruit and a bad tree bad fruit, and you know each by what it produces. Jesus then turns the image on the human heart and mouth: "out of the abundance of the heart, his mouth speaks." The word translated "abundance" pictures something overflowing, a container so full it spills over the sides.
The claim is that speech is diagnostic. What comes out of a person's mouth, especially when they are not being careful, reveals what has been quietly collecting inside. Jesus calls it "treasure," the heart's stored-up supply, and he says the supply eventually shows itself in words. This is less about a single careless sentence than about a pattern, the steady drift of what someone talks about, praises, jokes about, and repeats. The mouth files a report on the heart's contents.
The verse cuts two ways. It is a warning, because our words expose us more than we would like. It is also an invitation, because it locates the real work upstream, in what we allow to fill the heart in the first place. Change the speech by changing the treasure. Jesus treats the tongue as a window, and what it shows is never random; it is the overflow of a whole life.