“Matthew speaks of the ‘Kingdom of heaven’ where Mark and Luke have ‘Kingdom of God’. The meaning is the same in each case. Behind the Matthaean usage lies the Jewish linguistic rule of not using the Name of God, nor even the conceptual denomination ‘God,’ out of reverence for the greatness of this word. One spoke of him only by circumlocution. ‘Heaven,’ then, is simply a periphrasis for ‘God.’ This is an important point, because it shows that Matthew is not concerned, any more than are Mark and Luke, with something which is primarily in the world beyond. What is at stake is not the beyond, but God, in his personal activity. This observation gains in force if we add that in Jewish usage, the term basileia, normally translated ‘kingdom,’ does not signify a sphere of governance, but an active reality like our words ‘reign’ or ‘command.’ Thus the phrase ‘the Kingdom of God ’ points to God’s rule, his living power over the world. Following Joachim Jeremias, the affirmation that ‘the Kingdom of God is at hand’ can be paraphrased “God is close.’ First and foremost Jesus is speaking not of a heavenly reality but of something God is doing and will do in the future here on earth.”
-Joseph Ratzinger (Pope Benedict XVI) in Eschatology: Death and Eternal Life