It's not presumptuous at all. If you're a Christian, this is what you believe at the very basic level. Yes there are theological debates regarding other subjects, but these core beliefs are not disputed by those who truly understand Christianity and refer to themselves as Christians.<quoted text>
You are being presumptuous that ALL Christians stick to those "core" beliefs.
You are also being presumptuous that all non-Christians do not stick with their "core" beliefs.
I'll put it another way:
If your core beliefs do not include all of these, then you're not Christian.
Atheists do stick with their core beliefs. I didn't say they didn't. I said atheists and those who adamantly reject God are pushed to further and further lengths to "prove" their case. We stick to Tacitus and Josephus. We stick to the canonical gospels. It's not because we don't think, but that these are true and consistent with the available evidence. If I really was bent upon "proving" something for my own satisfaction, why would I stick to sources that atheists deny as having evidential value? I stick to Tacitus because Tacitus has historical truth. I stick to the canonical gospels because there is historical truth to it. It's just that simple.
Why complicate it?