Jesus waited until He was alone with His disciples and the twelve apostles to answer His disciple‚Äôs questions on His use of parables. Just as God the Father revealed the knowledge of who exactly Jesus is to Peter (Matthew 16:17), the same holds true for the Words and parables of Jesus. Those who earnestly seek God will find Him. Peter was earnestly seeking God and He found Him in Christ Jesus. What Jesus is saying here is not an intention to deny anyone the Gospel. Many would exhibit hostility toward Jesus and God the Father as well. Those who did not want God and sought to malign Him would be denied access to the Kingdom of God.
The Kingdom of God was a mystery in the sense that until the New Testament and the appearance of God on earth in Jesus the Kingdom was not fully known. These characteristics of God‚Äôs earthly Spiritual Kingdom were not revealed in the Old Testament.