Luke 17:20-37 also seems to support the idea that Jesus was trying to tell people the kingdom was spiritual, not physical. The kingdom as a concept wasn't some novel idea, either. Jesus was claiming he was the fulfillment of the messianic prophecy in Judaism. He was reinterpreting the prophecy, though, as a spiritual rather than literal liberation.

Tangential, but you can interpret the anti-christ in christian belief to bring the alleged kingdom, as a sort of anti-fulfillment of the prophecy.