All religions have mythology which often gets completely absorbed and becomes accepted as historical. The mythology varies. But what all religions do have are narratives that can be seen as either mythologies or invention. Religions are often little more than literature or constructed upon literature.

The early Sumerian religion was based on natural forces, storms, floods, sun, etc, but soon became individualised. This was achieved by scribes who communicated with each other, building up the god's they'd invented personalities and life stories. These grew to become overarching mythologies and remain evident, as does the process, in the present day.