I wanna say monotheistic because of Zoroastrianism, this religion out of ancient Persia. It’s been around for millenia.
But then there’s Hinduism and some of the ancient Chinese religions. Those are polytheistic.
You have the Old Testament proto-Judaism which practiced many idolatrous sacrifices, polytheistic in a sense.
I don’t know. It’s a good question and makes you think.
The Old Testament “proto-Judaism” was a corruption of the primordial religion of Adam and Eve. This is why Moses was given the First Commandment, “I am the Lord your God. You shall not have any gods before me.”
The basis of all religion is the spiritual side of human nature, which is drawn to seek out and understand God. The Bible documents the history of the “Chosen People”, who were set apart as an example for the nations, but were not necessarily the only nation with the correct religion. In scripture, there are hints that other nations worshiped the same God, such as the priestly King of Salem, Melchizedek, giving an offer of bread and wine on behalf of Abraham.
Many formal religious systems devolved into polytheism before written records were kept. I suspect if records existed, all peoples at one time believed in one God. As it is, many polytheistic religions still have a king of gods (although the king of gods can change based on which nation conquered which). Incidentally, our God even used this mythologies of successive gods to His advantage, leading the Israelite nation to numerous miraculous victories, and stumping the Egyptian magicians. Our God demonstrated in every manner that he was the true god!
Zoroastrianism is an interesting case, because it seems that the polytheistic pantheon was whittled down to a supreme being or intelligent creative force. Some of the corrupted beliefs were whittled away to form a truer, though imperfect religion. The religion even discerned the coming of a savior figure.
The bible tells us that the three wise man discerned the nativity of our lord by reading the stars. It is therefore, not impossible that the ancient Iranians also independently discerned the coming of Jesus, written into the fabric of the universe, though they must have misread many of the details.