The first parable in this collection in Matthew 13 includes a reference to “the secrets (or mysteries) of the kingdom of heaven” (v. 11). This expression also occurs in Luke 8:10 and in Mark 4:11-12 where the singular “secret” is used rather than the plural form used in Matthew and Mark: He told them, ‘The secret of the kingdom of God has been given to you. But to those on the outside everything is said in parables so that, “they may be ever seeing but never perceiving, and ever hearing but never understanding; otherwise they might turn and be forgiven!”’(quoting Isaiah 6:9). Mark’s wording suggests a single “secret” while the others suggest a secret which has several aspects. What is this secret, or mystery?
In Daniel we find the concept of God revealing His secrets to human beings. God grants the king a dream that was meaningless to him but whose meaning was revealed in a vision to God’s servant (Daniel 2:19). Paul understood “mysteries” to be revealed secrets (Romans 16:25-27) – things which had been hidden for long ages but now revealed to God’s people. The secret is proclaimed to everyone, even though it is understood only by believers. That the kingdom would come was no secret, but how it would come was not previously revealed. This secret is revealed in this series of parables in Matthew 13, and elsewhere in Jesus' teachings.
From Jesus' teachings about the kingdom we learn that the Old Testament prophets had only part of the picture. They expected the present age - "the kingdoms of men" - to end with the coming of the Messiah, and the Messianic Age would be paradise restored. What they didn't know was that there would be two comings of the Messiah, and that His first coming would be quite different to what they expected. Jesus came as a King and began His rule at His first coming.
Christadelphians generally have concentrated on the Old Testament prophets for their understanding of the kingdom, rather than the teachings of Jesus. Consequently their expectations are similar to the Jews of Jesus' time (or before) and they have failed to understand the "secrets of the kingdom" which were revealed by Jesus. This is a great pity because as a group Christadelphians have realised that the kingdom is central to the Gospel, but by neglecting the present reality of the kingdom they have missed out on many of the benefits which Jesus said would come "in this age" to those who embraced His teachings.