“Then Jesus said, ‘Whoever has ears to hear, let them hear.’ When he was alone, the Twelve and the others around him asked him about the parables. He told them, ‘The secret of the kingdom of God has been given to you. But to those on the outside everything is said in parables so that, ‘they may be ever seeing but never perceiving, and ever hearing but never understanding; otherwise they might turn and be forgiven!’”
Mark quotes this to summarise the message of the Kingdom parables.
The mystery of the Kingdom is the coming of the Kingdom into history in advance of its final manifestation. It is, what theologians call, "fulfilment without consummation." This is the truth illustrated by the several parables of Mark 4 and Matt 13.
The word ‘mystery’ suggests there are secret plans, thoughts, and activities of God that are hidden from human reason and must be revealed to those for whom they were intended.
However, the mystery is proclaimed to all even though only those who believe understand it. All are summoned to faith; but only those who respond are shown to have spiritual perception and understanding (i.e. only to those who have been "given the mystery" as Mark 4:11 says.)
The same can be said of Peter in Matthew 16.
Matthew 16:17 “Jesus replied, “Blessed are you, Simon son of Jonah, for this was not revealed to you by flesh and blood, but by my Father in heaven.”
Jesus is clearly indicating that one must receive supernatural illumination to understand the truth that Jesus is the Son of God. Although the mystery is proclaimed to all, not all are "given" this understanding. "Flesh and blood" is incapable of revealing it.
In the parable of the four Soils Jesus said the Kingdom had come upon humankind but not for the purpose of shattering evil. It is now accompanied by no display of irresistible power. Rather, the Kingdom in its present working is like a farmer sowing seed. It does not sweep away the wicked (or none would have hope).
It seems that only those whose soil had first been prepared by God would later receive the seed and bear fruit. The farmer must sow the seed in ground that has been broken up and since good soil is not its natural state it must be prepared before scattering seed. In other words, unless God is the one who first makes our heart of stone into a heart of flesh, we will not receive the gospel message.
“Jesus told them another parable: “The kingdom of heaven is like a man who sowed good seed in his field. But while everyone was sleeping, his enemy came and sowed weeds among the wheat, and went away. When the wheat sprouted and formed heads, then the weeds also appeared.
“The owner’s servants came to him and said, ‘Sir, didn’t you sow good seed in your field? Where then did the weeds come from?’
“‘An enemy did this,’ he replied.
“The servants asked him, ‘Do you want us to go and pull them up?’
“‘No,’ he answered, ‘because while you are pulling the weeds, you may uproot the wheat with them. Let both grow together until the harvest. At that time I will tell the harvesters: First collect the weeds and tie them in bundles to be burned; then gather the wheat and bring it into my barn.’”
Jesus affirms that in the midst of the present age, while society continues with its mixture of the good and the bad, before the coming of the Son of Man and the glorious manifestation of the Kingdom of God, the powers of that future age have entered into the world to create ‘sons of the kingdom,’ those who enjoy its power and blessings. The kingdom has come but society is not uprooted. This is the mystery of the Kingdom. The Kingdom has come into history but in such a way that society is not disrupted. The children of the kingdom have received God's reign and entered into its blessings. They must continue to live in this age. Intermingled with the wicked in a mixed society. The Kingdom that is present but hidden in the world will yet be manifested in glory.
“He told them still another parable: ‘The kingdom of heaven is like yeast that a woman took and mixed into about sixty pounds of flour until it worked all through the dough.’”
The parable teaches that one day the Kingdom of God will rule over all the earth, but has now entered into the world in a form that is hardly perceptible. The Kingdom of God is destined to permeate all human society until all the world is transformed by a process of gradual penetration and inner change. One day the Kingdom will prevail to such an extent that no rival sovereignty exists. The entire mass of dough becomes leavened.
In Jesus’ day and age, the mighty irresistible character of the eschatological Kingdom was understood by all Jews and would mean a complete change in the order of things. The present evil order of the world and of society would be utterly displaced by the Kingdom of God.
But Jesus’ ministry appears to have initiated no transformation like that. He preached the presence of the Kingdom but the world went on as before. How then could this be the Kingdom? Jesus’ reply was that when a bit of yeast is put into a mass of flour, nothing seems to happen. The leaven even seems to be engulfed in the flour. Eventually something does happen and the result is a complete transformation of the dough.
This idea of a gradual progression was unheard of by the Jews at the time but Jesus reiterated it again and again. No one could have guessed that Jesus’ small band of disciples had anything to do with the future, glorious Kingdom of God. However that which is now present in the world is indeed the Kingdom itself. This is the mystery, the new truth about the Kingdom.