Christians, is there such an expression as "the mystery of the Kingdom of Heaven"?

I know that Matthew talks about the Kingdom of Heaven and the other gospels talk about the Kingdom of God. I also know both phrases refer to the same thing. But someone asked me to explain "the mystery of the Kingdom of Heaven" and I am at a complete and utter loss to know where to start! What... show more I know that Matthew talks about the Kingdom of Heaven and the other gospels talk about the Kingdom of God. I also know both phrases refer to the same thing. But someone asked me to explain "the mystery of the Kingdom of Heaven" and I am at a complete and utter loss to know where to start! What mystery?

Is this an expression used in Catholicism? Any insights most welcome!
Update: Great answers! Thanks, everybody. This is a genuine request for information so I can respond to an assignment. While acknowledging that Jesus came to save the lost sheep of Israel, it's worth checking out Luke 2:25-40 when Simeon said this about the 8 day old Jesus: "For my eyes have seen your... show more Great answers! Thanks, everybody. This is a genuine request for information so I can respond to an assignment.

While acknowledging that Jesus came to save the lost sheep of Israel, it's worth checking out Luke 2:25-40 when Simeon said this about the 8 day old Jesus: "For my eyes have seen your salvation, which you have prepared in the sight of all people, a light for revelation to the Gentiles and for glory to your people Israel." Luke, as a Gentile, emphasises the truth that salvation was offered for the Gentiles as well as for Jews, but Simeon is quoting from Isaiah 42:6. Just a thought!
Update 2: Helpful links from Autumn and I am about to read the link from Amp Man. What a fascinating subject - I'm so glad I asked this question and I appreciate everyone's time and input.
Update 3: The reason I think kingdom of heaven and kingdom of God are the same is explained in these two links:

http://www.gotquestions.org/kingdom-heav...
http://www.gotquestions.org/kingdom-of-G...
Update 4: Clearly, as everyone has pointed out, there is such an expression as "the mystery of the Kingdom of Heaven", and there are differing views on what the Kingdom of Heaven is. However, when I asked, 'what mystery?' I didn't expect a debate on the difference between the Kingdom of Heaven and the... show more Clearly, as everyone has pointed out, there is such an expression as "the mystery of the Kingdom of Heaven", and there are differing views on what the Kingdom of Heaven is. However, when I asked, 'what mystery?' I didn't expect a debate on the difference between the Kingdom of Heaven and the Kingdom of God! My fault for mentioning it, I guess.

One person gave a really good explanation of Jesus' words to his disciples in Matthew 13:11 and Mark 4:11 when they asked why Jesus spoke in parables to the crowds but not to them. This answer shows how the mystery is revealed, and to whom. This is a vitally important point.

With regard to the differences between the expressions Kingdom of Heaven and Kingdom of God, as spoken by Jesus and contained within the New Testament, I think this deserves a fresh question, aimed specifically at the who, what, where and when aspects of God's Kingdom. Please feel free to copy and paste your posts to my next question, and forgive me for not being clear.
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