In what the Bible calls the "Ancient World" which was the world before the Flood we can assume that many knew of Elohim (God). This was evidenced by the line of Adam to Noah by notable characters such as Enoch (Gen. 5:22). However, the Ancient World, by in large, rejected God as evidenced by the following:
(1) Many had sexual relations with demons and spawned the Nephilim.
Gen. 6:1-4 When human beings began to increase in number on the earth and daughters were born to them, the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. Then the Lord said, “My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.” The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.So the Nephilim are said to have been the spawn of the "sons of God" which is referring to fallen angels whom God created (Gen. 6:4). The Nephilim were the part demonic, part human spawn of those demons and the "daughters of humans". So humans had already rejected God, starting with Adam and Eve, and only a few in the line of Adam were faithful to Him, though they did pass down the knowledge of Elohim.
(2) The ancient world was characterized by violence and perversion such as will not be seen again till the end times (the times we are living in).
Gen. 6:5-7 The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. The Lord regretted that he had made human beings on the earth, and his heart was deeply troubled. So the Lord said, “I will wipe from the face of the earth the human race I have created—and with them the animals, the birds and the creatures that move along the ground—for I regret that I have made them.”So mankind had rejected God all the way from the beginning in the Ancient World which was why they were condemned and destroyed by the Flood. They rejected God in favor of demons and their own lusts. So it is a misnomer to state that the one true God was being worshipped in the ancient world.
Luke 17:26 “Just as it was in the days of Noah, so also will it be in the days of the Son of Man.
After the flood it was the line of Noah who carried on the knowledge of God. We see this through the accounts of Noah, Shem, Job (Jobab in the line of Shem, Gen. 10:29), Terah and Abraham in the Bible which were passed down either in written form or orally to Moses who wrote them down for the people of Israel. However, not long after the Flood Noah's descendants began to reject God and attempted to establish a one world government known for the worship of Nimrod and his wife Semiramis. Nimrod may well have been a Nephilim himself because we know from Gen. 6 that the Nephilim were also seen after the flood. He was called "a mighty hunter before the Lord" (Gen. 10:9) and depictions of him carved in stone from that time found in Turkey show a man approximately twice the height of normal men. See my article on Nimrod here. We know of the Nephilim after the flood because the spies sent out from Israel reported seeing the sons of the Nephilim in Canaan.
Numbers 13:33 We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them.”God confused the tongues of those at Babel because they had disobeyed the Lord and were staying together, worshipping a male and female as gods and building them a ziggurat so they could be close to the heavens. This is because they beleieved Nimrod to be the sun (after his death it was said that he "became" the sun) and Semiramis to be the moon (Semiramis was also called the "Queen of Heaven") (http://www.torahandspirit.com.istemp.com/babylon.htm). This is where, I believe, the worship of the "Queen of Heaven" originated, which the Bible details. (Jer. 7:18; 44:17-19 & 44:25) Later they continued to be worshipped in Babylon as Marduk and Astarte, as well as in Canaan as Baal and Asherah. So when God forcibly sent everyone on their way from Babel they did not go out with the knowledge of God (Elohim) but of the false religion of polytheism they had devised along with the worship of created things called "Animism". This is what we see today which lays the basis for all Gentile cultural false religions. We know that the Bible teaches that the Gentiles "did not know God" over and over so those who have tried to prove that all the cultures from the beginning have all been based on monotheism such as Wilhelm Schmidt in his "Origin of the Idea of God", Don Richardson in "Eternity In Their Hearts", Daniel Kikawa in "Perpetuated in Righteousness", and many other World Christian Gathering on Indigenous People (WCGIP) advocates as well as Emerging Church, New Apostolic and liberal theologians, are dead wrong. They need to base their findings on what the Bible says over and over about the Gentiles not knowing God instead of starting from a false, badly researched premise.
Gal. 4:8 Formerly, when you did not know God, you were slaves to those who by nature are not gods.And yet we have many people out there trying to justify all the cultures of the world, putting them on equal footing with Israel, to whom God exclusively revealed Himself by Name (YHWH-"I Am That I Am" Ex. 3:14). These ideas from Richardson et. al. bascially constitute radical Replacement Theology on steroids. That is because they are trying to make all Gentile cultures into their own seperate trees when the Bible says that any true believer is a branch off the tree of Israel.
1 Cor. 1:21 For since, in the wisdom of God, the world through wisdom did not know God, it pleased God through the foolishness of the message preached to save those who believe.
1 John 3:1 How great is the love the Father has lavished on us, that we should be called children of God! And that is what we are! The reason the world does not know us is that it did not know him.
Rom. 1:28 Furthermore, since they did not think it worth while to retain the knowledge of God, he gave them over to a depraved mind, to do what ought not to be done.
1 Thess. 4:5 ...not in passion of lust, like the Gentiles who do not know God...
Ephesians 2:12-13 ...remember that at that time you were separate from Christ, excluded from citizenship in Israel and foreigners to the covenants of the promise, without hope and without God in the world. But now in Christ Jesus you who once were far away have been brought near through the blood of Christ.
2 Thess. 1:8 He will punish those who do not know God and do not obey the gospel of our Lord Jesus.
Rom. 11:17-18 If some of the branches have been broken off, and you, though a wild olive shoot, have been grafted in among the others and now share in the nourishing sap from the olive root, do not consider yourself to be superior to those other branches. If you do, consider this: You do not support the root, but the root supports you.The teachings of the WCGIP and others are creating a new world order that is a demonic lie and those who follow them end up worshipping false gods. Bible translation societies have also followed this paradigm and have placed the names of foreign gods in the Bibles of various languages, thus "blapshemizing" the Bible.
The truth is that man has rejected God from the beginning and after Babel took false religion to the four corners of the globe.
Here is just a partial list of some of the oldest cultures after the flood which had a male & female deity at the apex of their pantheon of gods.
Babylon: Marduk & Astarte (continued worship of Nimrod &
Anunnaki: Elil & Ishtar (see Babylon above) http://ancienthistory.about.com/library/weekly/aa102197.htm
Canaan/Ugaritic: Baal & Asherah http://home.comcast.net/~chris.s/canaanite-faq.html
Sumerian: An and Ki http://home.comcast.net/~chris.s/sumer-faq.html
Hitite: Kumarbi & Hannahannas http://ancienthistory.about.com/library/weekly/aa102197.htm
Egyptian: Amen & Mut http://socsci.colorado.edu/LAB/GODS/
Persia: Ahura Mazda & Spenta Armaiti (the earth goddess) (Source)
Mesopotamia: Enlil and Ninlil https://en.wikipedia.org/wiki/List_of_Mesopotamian_deities
Ninevah: El & Athirat (http://ancienthistory.about.com/library/weekly/aa102197.htm)
Hurrian: Teshub & Hebat https://en.wikipedia.org/wiki/Hurrian_deities#Religion
Lydian: Baal & Anat https://en.wikipedia.org/wiki/Anat
India: Vishnu & Shiva https://en.wikipedia.org/wiki/Hindu_deities
Armenia: Aramazd & Anahit https://en.wikipedia.org/wiki/Armenian_mythology#Pantheon
Pre-Islamic Arabia: Hubal & Manat https://en.wikipedia.org/wiki/Arabian_mythology
Ossetian: Huycau & Satana https://en.wikipedia.org/wiki/Ossetian_deities
Berbers: Isis & Set https://en.wikipedia.org/wiki/Berber_mythology
Greek: Zeus & Hera http://www.csun.edu/~hcfll004/zeusgirl.html
Roman: Jupiter & Juno http://www.gwydir.demon.co.uk/jo/roman/
Celtic: Rosmerta and ‘Mercury’, Nantosuelta and Sucellos, Sirona and Apollo Grannus, Borvo and Damona, or Mars Loucetius and Nemetona. https://en.wikipedia.org/wiki/Celtic_deities
China: Nüwa & Fuxi https://en.wikipedia.org/wiki/Chinese_mythology
Japan: Isanagi & Izanami https://en.wikipedia.org/wiki/Japanese_deities
Korea: Cheonjiwang & Queen Baji https://en.wikipedia.org/wiki/Korean_deities#Supernatural_beings
Aztec: Toci & Tlazolteotl https://en.wikipedia.org/wiki/Mother_goddess
Maori: Puhaorangi & Tangotango (no mention of 'Io) https://en.wikipedia.org/wiki/List_of_M%C4%81ori_deities
Hawaiian: Kane & Na' wahine (Keawe became both Kane (male god) and Na' wahine (female god) http://www.huna.com/gods_diagram.html
I could cite many, many more. But you get the point.
In conclusion, this puts the matter to rest about the claims that the religions of the world have always been monotheistic the further back in time you go. Hogwash! People who make these claims need to learn from anthropology, archeology and especially from the Bible!