In The End What Men Are Is Revealed In What They Produce By Their Lives (6:43-44).
a a For there is no good tree which brings forth corrupt fruit (Luke 6:43 a),
b b Nor again a corrupt tree which brings forth good fruit (Luke 6:43 b).
c c For each tree is known by its own fruit (Luke 6:44 a).
b b For of thorns men do not gather figs (Luke 6:44 b),
a a Nor of a bramble bush gather they grapes (Luke 6:44 c).
d d The good man out of the good treasure of his heart brings forth that which is good (Luke 6:45 a),
e e And the evil man out of the evil treasure brings forth that which is evil (Luke 6:45 b),
dde e For out of the abundance of the heart his mouth speaks (Luke 6:45 c).
e e And why do you call me, Lord, Lord, and do not the things which I say? (Luke 6:46).
For there is no good tree which brings forth bad fruit,
Nor again a corrupt tree that brings forth good fruit.
For each tree is known by its own fruit.
For of thorns men do not gather figs,
Nor of a bramble bush gather they grapes.
Jesus now emphasises that the test of what we are is the fruit that we bear. This applies to all who read these words. This is what salvation is all about. It is in order to produce fruit-bearing trees. Jesus is saying that a man will be revealed as what he is by what men behold in his life. If he is a genuine Christian, ‘a good tree', he will bring forth good fruit and not bad fruit. Whereas those who are corrupt trees, and therefore not Christians, will not produce good fruit but bad fruit. Every tree will be known by its fruit. Jesus is saying, ‘Show me a Christian whose life has not changed for the good, slow though the process may be, and I will show you a man or woman who is not a Christian.'
Our lives, says Jesus, should be producing good fruit, the equivalent of figs and grapes which delight man's heart. But if we are not producing such fruit then we are simply revealing ourselves to be brambles and thorns. And what fruit should we be producing? ‘The fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faith, gentleness and self-control' (Galatians 5:22).
But we should note that the point here is not that men are what they are and cannot be changed. The good tree here is a good tree because the Holy Spirit has made it so. It was not naturally a good tree. Christ has not come simply to develop good trees which do not need changing, He has come to seek and to save the lost and turn them into good trees. That is why He goes on to speak of the treasure that God puts in men's hearts.
Note the differences with Matthew 7:16. Both are clearly drawing from a different source in spite of similarities. There is absolutely no reason why one or the other should have arbitrarily altered the source of the fruit, whereas we can understand Jesus doing so at two different times depending on His surroundings.