Jesus said in Luke 6:43, "For a good tree does not bear bad fruit, nor does a bad tree bear good fruit."
During the course of his preaching, John the Baptist rebuked the multitudes that came to be baptized by him, using this same imagery.
Luke 3:8-9 - Therefore bear fruits worthy of repentance, and do not begin to say to yourselves, ‘We have Abraham as our father.’ For I say to you that God is able to raise up children to Abraham from these stones. And even now the ax is laid to the root of the trees. Therefore every tree which does not bear good fruit is cut down and thrown into the fire.”
John basically stated that the Israelites could not rely on their heritage for salvation. Yes, their lineage reached back to Abraham and the Israelite nation was the chosen people of God, but that was not enough to please God. John told them that they must "bear the fruits worthy of repentance", in other words, change their ways and live for God. Some action was required on their part.
This section of verses reminds me of James' writing concerning faith and works.
James 2:14-17 - What does it profit, my brethren, if someone says he has faith but does not have works? Can faith save him? If a brother or sister is naked and destitute of daily food, and one of you says to them, “Depart in peace, be warmed and filled,” but you do not give them the things which are needed for the body, what does it profit? Thus also faith by itself, if it does not have works, is dead.
Many are quick to point out that it is not works that save us, but the grace of God. While this is true, we cannot deny that, as Christians, there is a call for action on our part. We must not neglect Christian duties. We must strive to be active lest our faith wither as a dying tree.
Jesus tells us that His yoke is easy and His burden is light (Matthew 11:30). In submitting ourselves to God's will, we become Christians; We take on the work of Christ. This work becomes easier with time as we strive to become more Christ-like. Before long, I think we begin bearing that "good fruit" as naturally as a tree develops.
Galatians 5:22-23 - But the fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faithfulness, gentleness, self-control. Against such there is no law.
Do we have the love for our neighbor that our Lord commanded?
Do we abide in peace through a longsuffering attitude of kindness, goodness, and gentleness?
Do we exhibit self-control in all that we do?
Do we do all of the above faithfully?
Jesus Himself tells us that our actions define us.
Luke 6:43-44 - For a good tree does not bear bad fruit, nor does a bad tree bear good fruit. 44 For every tree is known by its own fruit. For men do not gather figs from thorns, nor do they gather grapes from a bramble bush.
What kind of fruit are you bearing?