"¿Esa religión cree en un Dios o en muchos dioses?"
Traducción:Does that religion believe in one god or many gods?
Por qué no me aceptan : does that religion believe in a god or many gods?
Porque decir "a god" se traduce como "un dios" en forma de articulo y no como un numero o cantidad, por eso hay que hacer uso de "one".
It might be better to say, that is just the way it is said in English. Who knows how it got that way, but that is the way it is. It is easier if you don't "hassle the petty stuff", which was the hippie way to say don't waste your energy worrying about the inconsistencies of the English language. There are too many of them.
Sometimes you just have to memorize. Some things don't follow a rule.
Spanish has one root -Latin - so it stays pretty consistent. English originally came from Latin and Germanic languages (and Greek, or course), so things were mixed up to start with. Then English kept adding from the languages of the cultures they "spent time in" (American Indian and Hindi for instance) and now there are so many different foreign language immigrants that new words and phrases just keep popping up. English is a very resilient language. It keeps changing with use, which is great for the language - it keeps it alive- but miserable for those trying to learn it.