From Al Mohler's radio programme - when he interviewed John Piper (& John MacArthur), Piper said the following about what is needed for the gospel to be the gospel and not another gospel. (NB typed from listening to the programme: listen to the whole thing here.) He sets it out clearly in four levels, which I found very helpful.
Mohler: "(In the US,) Isn't there the danger that some other gospel has infiltrated even what is known as evangelicalism?"
Piper: "Yes, and it can come either in the form of what you neglect, or it can come in the form of what you affirm wrongly. And there are at least, in my mind, four levels at which you can do both of those. I think the gospel has to be articulated - I mean, it's news, right - gospel means 'good news' so it's news, it's news, it's not ideology, it's news.
"So the first thing is, it's facts, and you can deny those: Jesus died and he rose again; those are facts, you can deny them, or you can leave them out.
"The second thing is, when he died and rose, he accomplished something between him and God. He bore sin, he provided a substitute sacrifice, he absorbed wrath. Those things were accomplished and you can deny that or you can affirm that and a lot of people are leaving out that - the whole doctrine of substitution is being elimimated by a lot of people or called 'divine child abuse' by a lot of people today.
"The third thing is: how do you obtain it? Works or faith alone? Some mingling of what's infused into you or what's imputed to you embraced by faith alone. You can get that all wrong and we are getting it wrong in terms of imputation.
"And the fourth is (and this is the one I'm caring the most about recently), the goal of all that is to get us to God, to enjoy him. So if we don't finally say the goal of the gospel is not restoration with Mom who's gone before, or everlasting golf, or escape from cancer, or getting out of hell, but fellowship with the living God enjoying him forever, then we're going to lose it.
"So those are the four pieces that have to be included, I think, to get the news right."
(HT: Justin Taylor)