There are four parts to the parable in Luke 6:39. Luke mentions that Jesus spoke to the people as he began concluding his sermon on the plain. The parts are: the blind leaders, the speck in the eye, the good and bad tree and the man’s heart (Luke 6:39-45). The parable stands out as a warning both to those who would follow after men and to those who trust in themselves that they are able to lead and teach men. Moreover, the parable contrasts good and bad teachers, showing what men should look for, before they choose which leaders or teachers they believe and follow.
Luke begins by pointing out that men have limited perception (Luke 6:39). It simply isn’t logical to believe what just any man says about the things of God, because, unless a teacher is gifted by God, that teacher isn’t capable of offering anything more than what most men are capable of finding out by themselves (cf. 1Corinthians 2:11). Jewish leaders have the benefit of the Law, but, if they put the traditions of men above the word of God, as Jesus showed they had been doing (cf. Luke 5:17-26, 39; 6:1-11), then they are no better than any other man (Romans 2:19-24).
Knowing this, the unavoidable fate of the disciple is locked into the maturity of his teacher. The disciple is simply unable to go beyond what he has learned from his teacher (Luke 6:40). In other words, Jesus is saying a teacher of God’s word is unable to pass on what he does not have. Therefore, it is inevitable that each and every one of his disciples will fail to grasp whatever the teacher does not know (cf. Matthew 23:13; Luke 11:52).
The problem is not mere ignorance. Ignorance can be overcome by the Spirit of God, The problem is a conscious choice was made (cf. Luke 5:39) to be satisfied with the status quo without even experimenting with anything new that showed great promise (cf. Mark 2:12; Luke 5:26). Almost always disciples will reject what their teachers reject. Therefore, the fate of every disciple is in the hands of his teacher (Luke 6:40).
With a little ANE humor, Jesus addresses the man who teaches traditions of men in the second part of his parable. Jesus claims that anyone with a log in his eye is unqualified to help his students remove their ignorance. First the teacher must get rid of his own great ignorance—his chosen ignorance, choosing men’s traditions over the word of God. Then he’ll be able to assist men in removing the ignorance into which they have been born. The difference between the ignorance of the teacher and that of the disciple is compared to a speck (in the disciple’s eye) and a log (in the teacher’s eye). The disciple’s ignorance is not as great as that of his teacher, because the teacher is unwilling to try something with great promise (cf. Mark 2:12; Luke 5:26, 39). At least the disciple is willing to learn. The teacher is not.
In part three of the parable (Luke 6:43-44), Jesus uses the tree as a metaphor for a man. Actually, the tree is put for plant life in general, because Jesus uses both the tree and the vine to represent good men, but the bramble or thorn bush is used for evil men. Clearly, all of these are not trees, and Jesus does at times use the tree to represent vegetation in general (Luke 13:19).
Jesus is saying that the blessed man who is a delight to the Lord is represented as a tree planted by the riverside, which yields his fruit in due season (Psalm 1:1-3). The corrupt tree, however, is represented by that which bears thorns. The same Greek word (G173 – thorns) is used in the Septuagint at 2Samuel 23:6 to represent the sons of Belial. Belial was a term used for men of evil character who showed themselves to be worthless individuals. Therefore, a good man (tree), who is blessed of the Lord and a delight to him will never show himself to be worthless. Rather, he will bear good fruit delighting both men and God. Nevertheless, the corrupt man (tree) who bears thorns is worthless and could never bear anything desirable (in a righteous sense) for either God or man.
Finally, in part four of his parable Jesus mentions the heart of man (Luke 6:45), which is the storehouse for that which he values (cf. Luke 12:34). Like the good and the corrupt tree, the righteous man brings forth goodness out of the treasure he holds in his heart, but the evil man brings forth evil from the same. Every man is known by the fruit of his mouth, because “out of the abundance of the heart the mouth speaks” (Luke 6:45). The good teacher will speak the word of God, blessing men and honoring God, but the evil teacher, while praising God in pretense, will ignore the word of God and preach the traditions of men.