Luke 6:39-42

From soniclight.org

In this parable the leader evidently represents a disciple and the led
someone the disciple is seeking to guide into the way of life. If the disciple
is blind, he will not be able to help other blind non-disciples find their
way. Both disciple and non-disciple will stumble tragically. On another
occasion Jesus called the Pharisees blind guides (Matt. 15:14). However
here He compared the disciples to them. The disciples could be blind
guides if they did not follow Jesus' instructions about loving (vv. 27-38)

Changing the figure momentarily Jesus compared a disciple of His to a
teacher. It is proverbial that a pupil cannot rise above his teacher in
knowledge. The fact that some pupils do excel their teachers is an
exception to the rule. The people the disciples would instruct in the truth
that Jesus taught them would normally advance no farther than the
disciples. This was especially true before the widespread availability of
books.224 Therefore it was imperative that the disciples pay careful
attention to Jesus' teachings about love and apply them. The progress of
the disciples' learners depended on it.

Jesus returned to the figure of limited perception (v. 39). It would be easy
for a disciple to criticize those he was instructing and fail to realize his
own faults since he was in the position of a teacher (v. 40). It would be not
only dangerous but hypocritical to try to help a learner overcome his
deficiencies without dealing with one's own failings first. If a disciple tried
to teach his learner the importance of loving as Jesus taught but did not
practice that kind of love himself, he could not remove his learner's
knowledge deficiency. His sin would be greater than his learner's
ignorance.
Thus Jesus stressed the importance of His disciples applying the truths He had taught
them before they tried to teach them to other people. Their failure to do so would make
them the spiritual equivalent of blind eye surgeons. They would be judging others but not
themselves (v. 37; cf. Rom. 2:1-3).