Paul never says anything about what we now call the Gospels (of Matthew, Mark, Luke and John), so his preaching must have taken place before they were put into writing. Scholar consensus has the Gospels starting around 70 CE with Mark, Matthew and Luke and ending with John in about 90 CE.
What is significant about the last Gospel, that of John?
Why don't the other Gospels say anything about this key tenet of the Christian faith?
There is no discussion in the Gospel of John of an upcoming "Kingdom of God." (Source: "Biblical Literature." Encyclopedia Britannica Online).
Why did the Gospels stop mentioning the upcoming Kingdom?
Did the early Christians change the message when it suited their interests in attracting converts and establishing their religion?