In these parables Christ reveals the secret plan of God for his kingdom during this gospel age (see Matthew 13:35). The hidden treasure is the nation of Israel, which God has called his "special treasure" (Exodus 19:5 and Psalm 135:4). Christ gave his life to redeem the world and, with it, his nation Israel. God will yet fulfil all his promises of blessing to the nation Israel because "the gifts and calling of God are irrevocable" Romans 11:29.  The great surprise to many of Christ's followers was that God's purpose for Israel in this age is that they be blinded and hardened so that the gospel may go to the gentiles.  This is the treasure still hidden in the field.