The mystery of the Kingdom of God

Notice these verses from Mark ch 4:

"......some sixty, and some an hundred.
9 And he said unto them, He that hath ears to hear, let him hear.
10 And when he was alone, they that were about him with the twelve asked of him the parable.
11 And he said unto them, Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables:
12 That seeing they may see, and not perceive; and hearing they may hear, and not understand; lest at any time they should be converted, and their sins should be forgiven them.
13 And he said unto them, Know ye not this parable? and how then will ye know all parables?
14 The sower soweth the word.
15 And these are they by the way side,........"

Note, v11, "Unto you it is given to know the mystery of the kingdom of God" (emphasis added). His disciples were able to know the mystery of the kingdom of God. Also, note John 3v3: "Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God." (emphasis added). Here, Nicodemus is told how the mystery is only perceived by those "born again".

In Mark ch 4, Jesus is telling his disciples of the sad fact that those "that are without" do not know the mystery of the kingdom of God.

Eastern's Revised Bible Dictionary comments on the word "mystery" used in the bible:

Mystery: The calling of the Gentiles into the Christian Church, so designated #Eph 1:9,10 3:8-11 Col 1:25-27 a truth undiscoverable except by revelation, long hid, now made manifest. The resurrection of the dead #1Co 15:51 and other doctrines which need to be explained but which cannot be fully understood by finite intelligence #Mt 13:11 Ro 11:25 #1Co 13:2 the union between Christ and his people symbolized by the marriage union #Eph 5:31,32 comp. #Eph 6:19 the seven stars and the seven candlesticks #Re 1:20 and the woman clothed in scarlet #Re 17:7 are also in this sense mysteries. The anti-Christian power working in his day is called by the apostle #2Th 2:7 the "mystery of iniquity."