What Do Christians Believe?
There's no real answer to that question, which creates a problem for anyone talking about Christianity, either pro or con. Aside from the concept that Jesus is the sacrificial lamb/deity that died to redeem everyone's sins, there's not a lot of common ground among the thousands of denominations that make up global Christianity.
From attitudes regarding the nature of the Bible's accuracy to widely differing opinions about such pithy topics as abortion and LGBTQ rights, someone saying "I'm a Christian" can mean almost anything by that statement. While similar things can be said for most major religions, the differences are greater in Christianity between the disparate denominations.
Christians disagree with one another on almost every level. From which faulty translation of the Bible is best to arcane topics like predestination and transubstantiation, disagreements are manifold. If you're discussing religions with someone who identifies as a Christian, you probably have almost no idea what that person actually believes on a wide range of issues. If you ask, you'll probably learn that many don't really know or are unwilling to tell you.
So, discussions where one disagrees with some aspect of Christianity aren't necessarily just arguments between a non-believer or atheist and someone who is religious. They might be arguments that could be made by any Christian discussing Christianity with another Christian from a different denomination. Christians have been arguing with each other for a couple of millennia. It's a favorite activity among them.
For atheists, it can seem rather simple. Atheists don't believe that any deities exist, or some variation on that theme. Christians do believe they exist, almost universally. At least one and maybe three that share some identity that varies, depending on which Christian you ask. On the bottom line, there's not a lot of room for discussion between those two extremes, really. You believe or you do not believe. There is no middle ground.
For atheists, there's just that one point. For Christians, the belief point is just a perch from which to take innumerable leaps of faith. Christians don't all believe the same things. Not even close. Christianity can be really complicated for atheists, since athiests don't have any of the nuances of belief to deal with. Christians do, so discussions often stray into nuance territory, gang all agly in the process.
I am an atheist. I cannot believe in deities and other supernatural stuff. Period. I can talk about that concept just fine with anyone. I'm an evidence-based kind of guy, and will insist on evidence which does not exist in my discussion. Whoever I'm talking to does not have such evidence. As it happens, I can also talk about a number of topics that divide Christians, but they are external to the essential topic regarding belief that deities exist and affect people's lives. I can't believe that, because there's no evidence of that.
I'm interested in the differences between various flavors of Christians. They're all rather tasty and worth nibbling on, on an intellectual basis at least. But, I'm an atheist. I once thought I believed that deities might exist, but I was just a kid then. Now, I don't, and can't. Nobody will convince me otherwise with the typical tautological arguments presented in discussion. Nor, do I suppose, will I convince anyone of my position. Atheism is arrived at on an individual basis, generally after considerable thought.
So, what's the point of discussing religion at all? It's interesting. That's all. So, here I am. Carry on.
= new reply since forum marked as read
I don't write those things any more. No time.
There is nothing that can be written that has no "wiggle room," as you cleverly put it. 
The whole "savior" thing is the one point on which Christians reliably agree, although they differ in the details of it in some cases. So, I didn't include many details.
an exercise in frustration. As you note, many are literalists themselves, until they are not, and the point at which they are not is variable. Trying to pin them down on such things is something like trying to add a polar bear to your collection of flying insects by pinning it to a piece of heavy paper. It's unlikely to be a success.
The whole literalists argument is a very convenient way to escape from a tight spot in an argument. "Well, you're just being a literalist on that, and I don't look at that literally." Asking what they look at literally will result in a request for some specific thing. If you supply a specific thing, they'll demur on their answer for some other reason.
Again, there is a fundamental difference. You have a simple disbelief in things supernatural. They believe in things supernatural. The gulf is too wide to cross, and nobody's going to part the waters to give you a dry path. Perhaps this will clarify:
It's obfuscated by the peculiarities of English grammar and negative-raising (using the Chomskian term).
My BIL current does not believe there is a god. It's no big deal to him. When his mother says she believes, it doesn't cut across something he actively believes. It's different from his belief, but doesn't deny his belief. Using pseudo formal terms, it comes out as [not {believe (exists <god>
}] versus {believe (exists <god> }.
Believe is a two-place predicate, subject of some sort + a proposition. Exists is just an operator. It's the believing that isn't there.
There are alternative placements of the negation.
[not {believe (exists <god> }] denies that one believes that there is a god.
[believe {not (exists <god> }] would be stating that one believes that god doesn't exist. It the formal logic I learned there were few differences between that and
[believe (exists <no god> ] . The denial that something exists and the affirmation that the thing is absent usually are a tautology.
My BIL for years believed there is no god. That comes out as denial not of belief in god, but belief that, in fact, there is no god. To object to this is to deny the object of a belief and to say that he's believing in something false.
It's the difference between somebody asking me if I'm Muslim and my saying, "No" versus saying "I think Muhammed was a Christian heretic." One's a denial of one's own belief; the other's the assertion of a belief and denial of the object of another's belief.
It may seem like a trivial point of little consequence, but to be honest, denying the trinity is of no greater practical import. It's like somebody who believes the Sun revolves around the Earth ... Really, if you're flipping burgers, doing brain surgery, drilling for oil, or programming Skynet, it really doesn't matter.
