The true followers of Jesus could understand the secret of the kingdom of God. They knew the kingdom had arrived (1:14–15). They knew that the Christ, the Son of God, was among them (1:1). To those on the outside, it seemed like riddles.
When Jesus answered the disciples’ question, he showed that he used parables to keep outsiders from understanding. This was so that they would be unable to repent and be forgiven.
The quotation from Isaiah 6:9–10 in the Greek Old Testament version implies this was a deliberate judgment on the hearers. Jesus taught in parables, and outsiders did not believe. This fulfilled what the Scriptures had said would happen.
The teaching of Jesus about the Kingdom of God was sensitive (it could sound like a political threat and worry Roman rulers). Because of this, teaching in parables helped make his message less likely to cause conflict.