When He wants us to be men of God, Christ calls us to be children. God calculates greatness on a different scale than we do. We cannot continue with earthly notions about greatness. Instead, we need to be converted and become as little children.
Christ taught Nicodemus in John 3 that the entrance to the kingdom of God is the new birth. Repentance was necessary for every one who came believing. Salvation was freely offered by grace through faith in the finished work of Christ on the cross. From this foundation, man now perceives the kingdom of God.