It makes the writer sound unintelligent and low in Conscientiousness. Perhaps this is because people with less of those things are less likely to use correct capitalization, leading to an association in my mind.

I'll admit, my first time through, my thought was: "maybe your atheist friend finds you hard to follow because you are stupider and less educated than you think you are, your reading speed due to haste & low comprehension, and your explanations & arguments are badly put or deeply fallacious".