Cut to the chase.
One thing that has come clear this far on the search is that in that passage in Romans Paul gives a huge gigantic clue. The reason he gives for not being ashamed of the gospel is that "it is the power of God."
For some reason I never caught this clue before. I always thought that Paul was saying something about the gospel, like I might say, "the apple is good for your health." "Good for your health" is something about the apple, it is not what the apple is. It says something about the appearances of how the apple interacts with things. It is, as it were, external to the apple. The apple is a tasty, sweet-tart fruit that grows on trees. I took Paul to be saying that the gospel, whatever that is, does God power stuff.
I have come to believe that the gospel is the power of God. The power of God is what the gospel is. The power of God is internal to the gospel like sweetness is internal to the apple.
But this isn't saying much, if it even makes sense. The beauty and hugeness of this clue really comes into focus when we come to see what it means in the context of the rest of what the gospel is.
So....what is the gospel?