What was the original one God religion?

Answer 1

As this is a 'religion and spirituality' category, I will answer via The Bible history written Pre-flood and Post-flood as they are the two major categories of man's history.

Man began in a one-on-one relationship with His Creator as many consider a monotheism religion today. In the beginning (Genesis) God created Adam and Eve who worshipped the One God or monotheism. More evidence in Genesis indicates that after the fall of man, his family still worshipped One God. It may seem something went awry during the time of Enoch and by chapter 6 of Genesis, men (apparently from Seth's line) were calling themselves 'sons of god' while they were taking 'daughters of men (appears to be Cain's line) inappropriately. They were calling themselves gods which would indicate the first diversion from monotheism to polytheism. God then destroyed the living creatures on the earth in the worldwide flood.

Post Flood or our current world, it appears the line of Ham's (Noah's son), via Cush aka Bel (Baal), was the first in this world to divert from worshipping the One God of creation as Noah and family had, back to polytheism. His son, Nimrod, furthered this polytheistic way having founded Babel or modern Babylon and the 'mysteries' or polytheistic practices associated therewith.

Worship of one God throughout history has been called by many names &/or ways of living of varied peoples which would indicate a common source to many who study this. Biblically, as the relationship with the Creator God of the Bible devolved, He set aside a 'chosen people' called the 'Congregation of Israel' to follow Him and His government as an example to all mankind. This too became corrupt and led to many being captured and lost to much of our current history. Only the tribe of Judah maintained this monotheistic religion and called it Judaism to this day.