Mark 4: 11 And he said to them, "To you has been given the secret of the kingdom of God, but for those outside, everything comes in parables; 12 in order that "they may indeed look, but not perceive, and may indeed listen, but not understand; so that they may not turn again and be forgiven.' "
What is the "mystery" or "secret" of the kingdom of God that Jesus speaks of, and how does this passage inform our understanding of the kingdom today?
A major key to identifying the “mystery” or “secret” of the kingdom referred to in verse 11 is found in verse 12 (a quotation from Isaiah 6:9-10): those who would not understand this mystery would not be forgiven. Those who have been given understanding of the mystery of the kingdom respond with repentance that leads to forgiveness; whereas those who have not been given understanding do not repent, and remain “outside” and unforgiven.
Mark introduces this association of the “mystery of the kingdom” with the forgiveness of sins from the very beginning of his gospel, when he equates the “good news,” or the Gospel, with the coming of the “kingdom.” This naturally leads us to consider the way that Paul uses the same word for mystery (Gr. Musterion) to refer to the mystery of the Gospel (cf. Ephesians 3:8-11; 6:19) and the mystery of Christ and His church (cf. Ephesians 5:32). I suggest that the “mystery of the Kingdom” is indeed the “mystery of the Gospel,” or “the mystery of Christ,” which “in former generations was not made known to humankind, as it has now been revealed to his holy apostles and prophets by the Spirit: that is, the Gentiles have become fellow heirs, members of the same body, and sharers in the promise [Israel’s hope] in Christ Jesus through the gospel” (cf. Ephesians 3:5-6).
The terms “kingdom of God” and “kingdom of heaven” are synonymous in Scripture. Therefore when we consider what it means to “enter” the “kingdom of God,” we should consider also what it means to enter “heaven,” or the presence of God, through faith in Christ. It is also helpful to consider the Scriptures upon which Jesus and the New Testament apostles were basing their understanding of the “kingdom.” For example, Psalm 145 associates God’s mercy (which he performed by the cross) with the power and glory of his kingdom. And Isaiah 52 connects the reign of God in “Zion” (which the writer of Hebrews equates with the church, cf. Hebrews 12:22-24) with Israel’s salvation and the proclamation of the Gospel.
If one enters this “kingdom” (which is “heaven”) through belief in the Gospel, then it is understandable that those who expected a kingdom inaugurated through political deliverance from Roman oppression in the first century were those who were blind to the mystery—to the kingdom’s true nature--and that this blindness prevented them from “turning again and being forgiven.” And it is no different today, as there are those who are looking for a physical fulfillment of the kingdom of God, or “Heaven on earth,” and are blind to its true nature, and missing the joy of its present reality.
 It is indeed appropriate, when reading and reflecting on the Scriptures theologically, to associate one writer’s teaching on the kingdom and the Gospel with another’s. Paul proclaims the Gospel that was the fulfillment of Israel’s hope, anticipated by her prophets. Mark states from the beginning that his Gospel (which he associates directly with the coming of God’s kingdom) is the fulfillment of those very same prophecies. Both Mark and Paul associate the “mystery” (of the kingdom, or the Gospel, respectively) with the forgiveness of sins. Mark and Paul both read the same Scriptures, and both saw Christ as fulfilling them.