So it finally ‘clicked’ for me what ‘the Gospel’ is. I wasn’t entirely convinced that it was ‘the Kingdom of Heaven is at hand’, as one of my friends preaches, because that phrase isn’t in the book of Acts (as far as I know).
But…the early Christians were martyred for declaring ‘Jesus is Lord’ (and by extension, ‘Caesar is not’), which is the same message told from our human perspective. The King says his kingdom is here. We declare Jesus to be king. Same thing.
Paul goes further, declaring ‘Christ and him crucified’, which explains who that king is and what he has done for us.
So there is agreement among the New Testament saints: Jesus is Lord, the Kingdom of Heaven is already here, and Christ has done all the work!