I just did a quick check and in the Gospels alone, the Kingdom is mentioned over 100 times and heaven (not in conjunction with the Kingdom; eg Kingdom of Heaven) is mentioned under 90 with most verses referring to it as the location of God, and hell is mentioned less than 20.
How many sermons do we hear on the Kingdom? Unfortunately, I think pastors teach on this important reality far less than what is needed. If they did, I think the people of God would live much, much differently and have much different expectations for life now and life in the future.
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After thinking about this for a couple of hours I wonder some of the reasons for not enough teaching on the Kingdom are possibly these:
1. A reaction by many conservative pastors to the liberals use of the Kingdom in the late 19th early 20th century.
2. An unbalanced futurist view of the Kingdom by many who would be pre-millennial or dispensational pre-millennial.
3. Not understanding what the New Testament means by "Kingdom" due to not understanding the Older Testament's anticipation of the Kingdom. (the OT and NT are one story!)
4. A pessimistic view of the present and future.
5. A view of Jesus that mostly relegates his role as Redeemer to the realm of saving souls, so Jesus is only understood in the scope of the salvation of man.
6. The idea that the Kingdom is only spiritual and too vague to really "nail down" what it means. (probably related to #3 on my list)
7. Kingdom is misunderstood as Church. Meaning, the Church is the Kingdom so if one teaches on the Church, they are teaching on the Kingdom, instead of what Scripture teaches as the Kingdom being something quite different (AKA Kingdom as God's reign and the Church as a steward or representative of the Kingdom).
That is all I can think of for now.