Are megachurches mainstream anything? I thought those were pretty fringe, like faith healers.
They're in a very strange spot. Kind of a mix of mainstream and fringe. Take Joel Osteen for instance, because I know the most about him, most of what he "teaches" is just feel-good self-help stuff. He mostly makes people feel better about themselves, without actually saying much specific about anything. This allows his "church" to have a broad appeal; not just to people who actually attend, but to people in other churches who might buy his books, or otherwise pay attention to what he's selling. Osteen has a huge influence on a lot of people. Similar to how Trump says stuff and lets his audience fill in their own version of the truth, Osteen does some very similar things when speaking. People hear what they want to hear when Osteen speaks, because most of the time he avoids saying anything but general positivity. He's like the generic happy feelgood version of Trump's generic vitriol. Speak vaguely enough with conviction, and a fanatical audience will fill in the gaps to suit their own preferred narrative.
But when he actually does say something that's not just fluff, or you look at the pastors he's most closely linked to (like T.D. Jakes), that's when you start getting into fringe stuff. Although, the prosperity gospel stuff that Jakes preaches would be mainstream if you could lump all prosperity gospel teachers into one group, that mostly doesn't happen because they differ a lot in specifics, and no one trying to scam their followers like that is willing to share if they can help it. Osteen's "church" really is about making money, and promising a better life to those who follow him (usually with a heavy implication that it will be a materially better life, and fast).
I think you could liken Megachurches' mix of mainstream and fringe to fad diets. The South Beach Diet, Atkins, juice fasts, etc. They're all fringe, and sometimes even denounced by mainstream medical opinions, but nearly everyone has heard of them, and even people who don't follow them are often influenced by what they've heard. They have a broad appeal, and good media coverage, so they seem like a higher percentage of the population is following them than is really the case.
