Rambam's History of Religion

The Rambam, in the first chapter of his Hilchot Avodat Kochavim, gives a brief history of religious practice beginning with the creation of the world through the birth of the Jewish people. The Rambam writes that at first man exclusively worshiped God. However during the days of Enosh (Breisht 5:6,9-11) the generation's belief system was corrupted by a mistaken theory. The people reasoned that God had placed the sun, the moon, stars and planets in the heavens because He held them in high regard. The people then reasoned further, that it would be appropriate to worship and praise the stars as a way of honoring Hashem; everyone knew that Hashem was the only God, however they felt that a king is honored when his most loyal servants are honored.
As the years went by false prophets emerged claiming that they were told by God to build temples and offer sacrifices to the sun, moon and stars. They also claimed to have been told to build images and idols of the constellations. As time went on, people began to claim that the idols and the sun and the moon and the stars were actual gods; no longer were they "servants of God", they themselves were gods. As time went on even further, the people forgot the One and true God entirely.
Rambam points out that along the way there were individuals who were able to recognize the existence of God, such as Noach, Shem, Eiver and Metushelach. However, no one was able to reach the heights that Avraham reached. Avraham, writes the Rambam, spent day and night contemplating the existence of a single God. He looked at the sun, moon and stars and believed that there must be a deity controlling them; they could not be without a leader. Finally, at the age of 40, Avraham came to the realization that there was only one God in the world. Shortly thereafter he began to spread the word of God to people of his town. He would engage in religious debates with them and would always emerge victorious. Avraham's attempts to convert people to monotheism caused him to be ridiculed by his fellow townspeople, eventually leading the king to throw him into a fiery furnace, prompting his family's move from Ur Kasdim to Charan.
Avraham successfully spread monotheism wherever he went and eventually passed on his position as God's chief advocate to his son Yitzchak who then passed it on to Yaakov. And the rest is history.