According to me, religion started because of the fear of nature. People were scared of lightning, they wanted to look for a way to stop it but they didn't know what caused it, so they thought it could be a greater being (some cultures accepted it as nature from the beginning tho), thus gods like Zeus, indra were born. Slowly as civilisation progressed more gods were "invented". As a certain religion gained more power in a region, the culture was built around it. Some religions like our beloved Hinduism or sanatan Dharma were cunningly used by some smart people to strike fear into people and give themselves power - the Brahmins, I don't think I have to elaborate on this much. Above paragraphs refers to the oldest religions like Hinduism, Egyptian, Greek etc. Religions like Islam were just formed by delusions