Paul Earnhart on Matthew 7:15-20:
Jesus’ analogy from nature—the tree is known by its fruit—drives home the point that kingdom citizenship is not a matter of appearance, but of being. People, like trees, produce the kind of fruit that their nature demands. Therefore, being a Christian is not simply a matter of doing something new, but of being something new. It is the kind of life which begins in the heart, at the center of the personality. That is the reason it is only produced by a new birth (John 3:3-5).
Some have attempted to follow Christ by adding some new dimension to their lives when it is the life itself which must be changed. You can tie grapes on thorns and figs on thistles, but they will not grow there. A wolf may wear wool, but he cannot produce it. The true child of the kingdom is different. As Jesus said, “out of his heart will flow rivers of living water” (John 7:38). Sin in all its manifestations begins in the heart (Matthew 15:19) and it is consequently in the heart and from the heart that a new kind of fruit must be borne.