What did Jesus Christ actually teach about the Kingdom of God/Heaven?
First, it is clear that he preached about it Matthew 4:17, Mark 1:14, Luke 4:43, 8:1, 9:11
He taught that the pursuit of the kingdom should be our priority Matthew 6:33, Luke 12:31; and that no second guessing would be tolerated Luke 9:62
He equated himself with the kingdom Matthew 12:28, Mark 1:15, Luke 11:20, Luke 17:20-21
He said that it was hard for wealthy individuals to enter Matthew 19:23-24, Mark 10:23-25, Luke 18:24-25
He said that the kingdom belongs to the poor Luke 6:20, Matthew 5:3 and persecuted Matthew 5:10
Jesus said that the most inconsequential person in the kingdom will be greater than John the Baptist Matthew 11:11, Luke 7:28
He said that it was easier for sinners to enter that kingdom than for religious leaders to do so Matthew 21:31; but that those who adhere to the commandments will be the greatest Matthew 5:19; and that those who wish to be a part of the kingdom must be more righteous than the religious leaders of Christ's day Matthew 5:20
He said that it is not the birthright of the Jews Matthew 21:43; and that people will come from all over the world to be a part of it Matthew 8:11, Luke 13:29
He said that it had been revealed only to those whom God had chosen to reveal it Matthew 13:11, Mark 4:11; and he told a parable about how different people react to the message Mark 4:1-20, Luke 8:10
Jesus compared the kingdom to the growth of a farmer's crop Mark 4:26-29
He also compared the kingdom to a mustard seed - pointing out that it starts out small and grows into something big Matthew 13:31-35, Mark 4:30-34, Luke 13:18-21
He compared the kingdom to a a farmer who planted good seed and an enemy who planted weeds, and that the two would be separated at harvest time Matthew 13:24-30, 36-40
He said that the kingdom will be manifested in power and glory (the transfiguration) Mark 9:1-12, Luke 9:28-35 He also said that the patriarchs and prophets would be in kingdom Luke 13:28
He said that the inhabitants of the kingdom will exhibit the innocence and humility of children Matthew 18:1-4, 19:14, Mark 10:14-15, Luke 18:16-17
He compared the kingdom to a nobleman going to a distant country to be crowned and return and settle accounts with his servants Matthew 25:14-30, Luke 19:11-26
Jesus compared the kingdom to a treasure or priceless pearl Matthew 13:44-46
He compared the kingdom to a fisherman sorting the good fish from the bad Matthew 13:47-50
He compared the kingdom to the owner of a vineyard who employed the people whom he chose to work for him and rewarded them how he wanted to reward them Matthew 20:1-16
He compared the kingdom to a king who prepared a great wedding banquet for his son and invited the guests Matthew 22:1-14
He also compared it to a polygamous marriage ceremony in which 5 of the brides were prepared and five were not, and the ones who were unprepared didn't make it into the marriage/kingdom Matthew 25:1-13
He said that the institution of the kingdom would be preceded by signs Matthew 24, Luke 21
He declared that one must be born again to see the kingdom John 3:1-5
He gave the keys of the kingdom to his disciples Matthew 16:19
Jesus said that the inauguration of the kingdom would involve rewards and punishments Matthew 25:31-46
Finally, Jesus Christ said that those who would overcome in this life would be allowed to share his throne Revelation 3:21; but he also said that it would be God's prerogative to assign positions under him Mark 10:35-40
Now, it is true that his disciples elaborated on these teachings, but it appears that these constitute the actual teachings of Jesus Christ on the subject of the Kingdom of God/Heaven.