When Jesus uses the image of the tree, he is drawing, as so often, on an ancient biblical picture. The first Psalm speaks of God’s true people like trees planted by streams of water, which will produce fruit at the right time, while the wicked are like chaff blown around by the wind. Jeremiah develops this picture (17.8), thinking of the tree that sends out its roots to look for the water it needs. Lent is a time when we should be doing that: sending out our roots to look for the water of life. The challenge of these verses isn’t simply one of learning to recognize true Christian teaching from false. The challenge is to become, ourselves, trees that bear good fruit, people who not only say ‘Lord, Lord’ when it suits us, but who apply ourselves to the much harder task of discerning and doing God’s will.
N. T. Wright