I’ve already mentioned—more than once—Greg Gilbert’s excellent primer, What Is the Gospel?
I’d also recommend his T4G breakout session on the same topic, which you can download or listen to below:
I found especially instructive the way in which Greg helps us to see how to keep the cross central as we preach and teach on the the kingdom of God:
The kingdom is an incredibly important theme in the Bible, and it’s good that evangelicals are thinking hard about it. But it seems to me that far too often when evangelicals start talking about the kingdom, there’s an almost reactionary tendency not to say much about the cross. It’s almost like it’s a different story, and we can’t figure out very well how the cross fits into this story of the kingdom. So we manage to create in our thought and conversation a rift between the cross and the kingdom, with cross over here and kingdom over there and everyone crouching on one side or the other of the chasm, sneering suspiciously at each other.
I don’t think the Bible leaves us with such a division, though. Here’s why: The only way into the Kingdom is through the Cross. Yes, Jesus came to inaugurate a kingdom which will one day be established with perfect justice and righteousness. But that is good news only because he also came to save a people from the wrath of God so that they could be citizens of that kingdom, and the way he did that was by dying in the place of those people for their sin. Jesus is not just King; He is Suffering King.
Put another way, it is the cross—and the cross alone—which is the gateway to the blessings of the kingdom. That’s how you put all this together. You don’t get the blessings of the kingdom unless you come into them through the blood of the King. Therefore if you preach a sermon or write a chapter on the good news of the kingdom, but neglect to talk about the cross, you’ve not preached good news at all. You’ve just shown people a wonderful thing that they have no right to be a part of because they are sinners. That’s why we never see Jesus preaching, “The kingdom of God has come!” No, it’s always, “The kingdom of God has come! Therefore repent and believe!” He didn’t just preach the coming of the kingdom. He preached the coming of the kingdom and the way people could enter it.
So by all means, preach about the kingdom, talk about Jesus’ conquest of evil, write about his coming reign. But don’t pretend that all those things are glorious good news all by themselves. They’re not. The bare fact that Jesus is going to rule the world with perfect righteousness is not good news to me; it’s terrifying news, because I am not righteous! I’m one of the enemies he’s coming to crush! The coming kingdom becomes good news only when I’m told that the coming King is also a Savior who forgives sin and makes people righteous—and he does that through his death on the cross. Ignore that, downplay it, shove it out of the center of the gospel, and you make the whole thing not good news at all, but a terrifying message of judgment to rebellious sinners.