The oldest religions have no human author; rather, sages and saints and guru's sensed the guidance of the divine in meditation and taught people how to live a human life and to rise above the animal instinct.
Early religions of mankind were animistic, they sensed there was a greater power, and made offerings to propiatate that greater power and to obtain a beneficial life. Ancient Hindus worshipped God as Fire, calling that god Agni. The American Indians worshipped animal spirits and had a Grandfather spirit, who was Grandfather to all. The Atzecs had sun worship of Quetzalcoatl, the Australian Aboriginals had Baiame and Rainbow Serpent, who created the Earth in the Dreamtime.
It seems that every society had some form of worshipping spirits. Over time, these became formalised into Religion.
Hinduism teaches that God reveals through sages and saints. Zoroastrianism, another ancient religion from Persia, taught that God was fire, and to worship the fire by casting all your bad thoughts into the fire itself.
- Another Answer
Because there were questions we couldn't answer. For example, in prevous times, humans did not know what stars were (other than strange glowing dots on the sky in the night). So they probably invented explanations for it, but most of which without any scientific research.
Some Religions start because people recognise that there must be something more to life, and that there must be some reason for us being here.
However, these questions can be explained by science, but are usually not considered enough.