His disciples asked him what this parable meant. He said, "The knowledge of the secrets of the kingdom of God has been given to you, but to others I speak in parables, so that, " 'though seeing, they may not see; though hearing, they may not understand.' (Luke 8:9-10)

As was reported in Matthew's gospel, the disciples do not understand the parable of the sower and seek an explanation. Jesus answers by quoting Isaiah and seems to differentiate between insiders and outsiders.

But then Jesus - in both Mark and Luke - adds:

"No one lights a lamp and hides it in a jar or puts it under a bed. Instead, he puts it on a stand, so that those who come in can see the light. For there is nothing hidden that will not be disclosed, and nothing concealed that will not be known or brought out into the open. Therefore consider carefully how you listen. Whoever has will be given more; whoever does not have, even what he thinks he has will be taken from him." (Luke 8:16-18)

So nothing is hidden from those who listen. Not just listen, but consider carefully how to listen. The Greek is blepo which is to discern with self-awareness and to use our deepest sources of understanding.

The gospel of Luke then inserts this brief set-piece. Mark reports the same exchange, but in an entirely different context and sequence.

Now Jesus' mother and brothers came to see him, but they were not able to get near him because of the crowd. Someone told him, "Your mother and brothers are standing outside, wanting to see you." He replied, "My mother and brothers are those who hear God's word and put it into practice." (Luke 8:19-20)

So the knowledge of the secrets of the kingdom of God requires careful listening, deep understanding, and application.