The short answer is no. The longer answer is . . . it depends on what you mean by “invent.”

One doesn’t have to look too far in the Torah to find individuals who recognized and served “the one G‑d.” Aside from the more well-known biblical personalities like Adam and Noah, we also find people like Enoch, about whom the verse states, “And Enoch walked with G‑d,”1 and Melchizedek, the king of Salem,2 who “was a priest to the Most High G‑d.3

What is more fascinating is the Midrashic story that Shem, son of Noah, and his great-grandson Eber (Eiver), actually set up a beit midrash, house of study, after the flood.4 Not much is recorded about this school, but we can glean some details from the Midrash and commentaries.

Yeshivat Shem V'Eiver—The Academy of Shem and Eber

What was studied at this academy?

In addition to learning about the oneness of G‑d and morality, they also learned the Torah laws as passed down from Adam, Seth, Enoch, Noah, Shem, etc.5 The fact that they had knowledge of the Torah laws (before the Torah was given) is attested to in the Torah itself, where we read that Noah brought two of each non-kosher species into the ark and seven of each kosher species. How did he know which were pure? Obviously, he had learned Torah.

Abraham himself eventually went to learn for a time with Shem. Indeed, the Midrash explains that Melchizedek, the king of Salem, who blessed and greeted Abraham after he had fought with the four kings and rescued his nephew Lot, was none other than Shem, son of Noah.

Isaac, too, is said to have gone to the academy of Shem and Eber to learn after the incident of the Akeidah, when Abraham almost sacrificed him to G‑d.6

Later, when Rebbeca felt great pains during her pregnancy and “went to inquire of G‑d,”7 it was to the academy of Shem and Eber8 (who were themselves great prophets9) that she went. She was informed about the twins she bore and how each one was destined to be the progenitor of a great nation.

Finally, the sages point out that if you calculate Jacob’s life, there are 14 years that are unaccounted for. They explain that according to tradition, after fleeing his brother Esau, he first spent 14 years studying in the academy of Eber (Shem had already passed away).10

This all leads to the obvious question: If there were a number of other individuals who believed in and served the one true G‑d, what was so special and unique about Abraham? And why do some call him the father of monotheism?

Abraham’s Uniqueness

Unlike Shem and Eber, who learned about G‑d from their own ancestors, Abraham, at least initially, came to the recognition of G‑d on his own. As Maimonides11 eloquently puts it:

Though [Abraham] was a child, he began to think [incessantly] throughout the day and night, wondering: How is it possible for the sphere to continue to revolve without having anyone controlling it? Who is causing it to revolve? Surely, it does not cause itself to revolve.

He had no teacher, nor was there anyone to inform him. Rather, he was mired in Ur Kasdim among the foolish idolaters. His father, mother and all the people [around him] were idol worshipers, and he would worship with them. [However,] his heart was exploring and [gaining] understanding.

Ultimately, he appreciated the way of truth and understood the path of righteousness through his accurate comprehension. He realized that there was one G‑d who controlled the sphere, that He created everything, and that there is no other G‑d among all the other entities. He knew that the entire world was making a mistake. What caused them to err was their service of the stars and images, which made them lose awareness of the truth.

Abraham was forty years old when he became aware of his Creator. When he recognized and knew Him, he began to formulate replies to the inhabitants of Ur Kasdim and debate with them, telling them that they were not following a proper path.

He broke their idols and began to teach the people that it is fitting to serve only the G‑d of the world. To Him [alone] is it fitting to bow down, sacrifice, and offer libations, so that the people of future [generations] would recognize Him. [Conversely,] it is fitting to destroy and break all the images, lest all the people err concerning them, like those people who thought that there are no other gods besides these [images].

When he overcame them through the strength of his arguments, the king desired to kill him. He was [saved through] a miracle and left for Charan. [There,] he began to call in a loud voice to all people and inform them that there is one G‑d in the entire world and it is proper to serve Him. He would go out and call to the people, gathering them in city after city and country after country, until he came to the land of Canaan—proclaiming [G‑d's existence the entire time]—as [Genesis 21:33] states: "And He called there in the name of the L‑rd, the eternal G‑d."

When the people would gather around him and ask him about his statements, he would explain [them] to each one of them according to their understanding, until they turned to the path of truth. Ultimately, thousands and myriads gathered around him. These are the men of the house of Abraham.

He planted in their hearts this great fundamental principle, composed texts about it, and taught it to Isaac, his son. Isaac also taught others and turned [their hearts to G‑d]. He also taught Jacob and appointed him as a teacher.

Thus, as the commentaries12 explain, yes, there were individuals who not only served the “one true G‑d,” but even at times went out to try to teach others to follow His ways, going so far as to set up an academy. So no, it would not be accurate to say that Abraham was the first monotheist.

However, Abraham not only came to the recognition of G‑d on his own, He, unlike Shem and Eber, was willing to sacrifice his life for his beliefs. Not only that, but he (unlike Shem and Eber who, for the most part, stuck to their own academy) took it upon himself to spread this knowledge of the true Creator to all people wherever he went. So although he certainly didn't invent monotheism, he was the first one to give his life for it and share it with the masses.