To you the secret of the kingdom of God has been given: but to those who are outside everything comes by way of parables, so that (as Scripture says) they may look, but see nothing; they may hear, but understand nothing; otherwise they might turn to God and be forgiven. (Mark 4:11-12)
For many people the parables are their favourite parts of the Gospels, and some, such as the Prodigal Son or the Good Samaritan, are well-known and loved by many. Instead of preaching a sermon, Jesus tells a story to make a particular point and let the lesson sink home. So it comes as a shock for us to hear from Jesus that his real purpose in using parables was to confuse people, so that his hearers who were not his followers might hear him but not understand him. We find parables in the Old Testament, and it was usual for rabbis to teach in this way. They were meant to make people think. They were never intended to be misunderstood, as Mark 4:12 seems to say. What is going on here?
We can easily see that the saying is an adaptation of Isaiah 6:9-10. God said to him, "God and say to this people:'Hear, but do not understand; see, but do not perceive.' Make the heart of this people fat, and their ears heavy, and shut their eyes; lest they see with their yes, and hear with their ears, and understand with their hearts, and turn and be healed."
Does this mean that Isaiah was told to prevent them from understanding his message so that it would be impossible for them to repent and escape the destruction that was coming? No, we cannot think that. What we must remember is that it was a Hehrew tendency to express the result of something as if it was a purpose. God is saying to him, "Go and deliver my message, but don't expect people to pay any attention to it. The result will be that they will refuse to pay any attention to it and so will be incapable of accepting it." This is exactly what happened to Isaiah. The effect (but not, of course, the purpose) of his preaching was exactly what is given in Isaiah 6:9-10. The verses quoted are just an exaggerated way of saying, "They will not listen."
The saying of Jesus here comes just after the parable of the Sower. His listeners would certainly have understood that Jesus was not talking about farming, but about his own message. He was urging them to have faith that in spite of great difficulties God would bring forth a great harvest, thirty, sixty, or even a hundredfold. Here he is talking to his committed followers who had come to understand at least something of God's love and forgiveness. There were many more, however, who did not understand anything and rejected Jesus. The prophets had faced the same problem, and that is why Jesus uses the words of Isaiah, because Isaiah had had the experience of preaching to people who simply did not want to know. They were spiritually deaf and blind.
So far from trying to confuse his listeners, Jesus is putting his finger on a problem that still faces the Church today.