"But Mark's point is different. The disciples are not given the gift to know, and what they have been given is in the singular, mysterion, not a secret but a mystery. This may be well the key word in Mark's narrative. One can scarcely miss the associations it suggests with the mysterium tremendum ac fascinosum. Jesus himself is the singular 'mystery of the kingdom,' and he is so as the Holy One. He is recognized fully only by God and other spiritual forces. He radiates an intense and fearful power. It is a power, furthermore, that at once attracts and repels, so that some are drawn to him and some reject him. Most of all, the mysterion resists understanding. It cannot be deciphered, controlled, or reduced to a formula. The mystery of the holy, even when revealed, remains beyond reach.
[Johnson goes on to describe Mark's portrayal of the disciples as both specially called to share in the ministry of Jesus and relentlessly ignorant of what his messiaship entails]
These literary observations suggest something of Mark's religious purpose in shaping the story of Jesus and the disciples in this fashion. Mark's readers would naturally, as we still do, identify themselves with the disciples. Mark therefore uses the relationship to teach his readers. The message is mainly one of warning against smugness and self-assurance. He seems to be saying, 'If you think you are an insider, you may not be; if you think you understand the mystery of the kingdom and even control it, watch out; it remains alive and fearful beyond your comprehension. If you think discipleship consists in power because of the presence of God, beware; you are called to follow the one who suffered and died. Your discipleship is defined by his messiahship, that is, in terms of obedience and service.'"
~Luke Timothy Johnson, The Writings of the New Testament: An Interpretation, 3rd. ed. (Minneapolis, MN: Fortress Press, 2010), 153.