In the 2010s I read this short story [in English] that I can't seem to find again. My vague impression was that it could have been written as early as the 1980s or '90s, but maybe it was more recent. I'm almost certain I read it online. And it may have been the first chapter of a longer work, but the ending made it a pretty complete-feeling standalone story.
The Setting: This is an alternate-history world where the Roman Empire accidentally stumbled upon some kind of electromagnetic effect that makes a person feel the presence of god in some way. (It may or may not have been a direct reference, but this reminded me of the purported "god helmet" effect.) A mention of the history of the tech implied that some local conjunction of magnetic rocks/something caused such biblical events as Moses with the burning bush, and after Roman investigation of the area they managed to turn it into a repeatable technology. In the present of the story, there are towers (like cell towers?) that emit the effect in a radius, so that everyone around feels the presence of god, and this constant "proof" of the divine reinforces the religion. (I don't think it was explicitly called/like Christianity in any way.) I also remember a (mag-lev?) train station where the protagonist scornfully observes a beggar/somebody laying about enjoying a tower's effect like a narcotic.
The Plot: We follow a Roman soldier-guy — a legionary, but maybe a higher-ranked officer — and he is very devout in their faith. (I have no idea about his/anyone's name.) He fights brutally and conquers more land/nonbelievers for the Empire, then either as a reward for his success or by his own volunteering or maybe as an accident of battle making it necessary, he is subjected to an experimental medical procedure that removes his ability to feel that "presence of god" effect that made him so devout before. I think he then does some soul searching and decides he will still believe in god even without proof, and redoubles his efforts and is even more brutal/effective in waging war. And the story ends with some kind of reveal that the scientists/leadership of the Empire who did the experimental procedure on him were trying to prove some kind of point, which the soldier's actions validated.
I'm amazed I remember this much detail… Too bad the title/author remains a total mystery!
The setting really stood out to me; especially the way faith is treated in this world where you can trigger the neurological feeling of a spiritual experience.
And the ending — it wasn't quite a "twist", but the protagonist's leaders/scientists did reveal something to him about his experiences being a successful experiment, but I'm not sure what. (That's why I'm asking this now!)