It is like a mustard seed that, when it is sown in the ground,
is the smallest of all the seeds on the earth.
But once it is sown, it springs up and becomes the largest of plants
and puts forth large branches,
so that the birds of the sky can dwell in its shade.”
-- Mk 4:26-34
The kingdom of heaven was inaugurated on earth by Christ. "This kingdom shone out before men in the word, in the works and in the presence of Christ" (LG 5). The Church is the seed and beginning of this kingdom. Its keys are entrusted to Peter.
-- CCC 567
Jesus' invitation to enter his kingdom comes in the form of parables, a characteristic feature of his teaching. Through his parables he invites people to the feast of the kingdom, but he also asks for a radical choice: to gain the kingdom, one must give everything. Words are not enough, deeds are required. The parables are like mirrors for man: will he be hard soil or good earth for the word? What use has he made of the talents he has received? Jesus and the presence of the kingdom in this world are secretly at the heart of the parables. One must enter the kingdom, that is, become a disciple of Christ, in order to "know the secrets of the kingdom of heaven". For those who stay "outside", everything remains enigmatic.
-- CCC 546