"In order for a man to understand with his heart the truth, which is by nature completely foreign to his heart, something first must 'be taken away' from his heart - namely that hardness which surrounds his heart and prevents his 'seeing', 'hearing', 'understanding'. The parables are designed to help make ... discernment possible; they do this by presenting the 'unspiritual man' with 'riddles' (Prov.1:6), calculated to crack him before he can crack them." - Robert L. Short: The Gospel according to Peanuts

Jesus often uses parables to hide the truth first, and then make it apparent. Here, the Parable of the Sower and the Seed is to "proclaim the Good News of the Kingdom of God" (Lk. 8:1 ), and even His disciples, so close to Him, had to ask for an explanation. The essential explanation is that God is merciful. Even though seeds falling on rocks and thorns are disastrous failures, the seeds (the word of God) will also produce an abundant harvest (Lk. 8:4-8) in those to whom are revealed the mysteries of the Kingdom of God.

Those with this knowledge are also aware that the word leads to salvation, and their mission is to persevere, despite any future persecution, in preaching that God is merciful and that Jesus makes God's rule present.