Hello,
For a long time I was under the impression that only Jews were saved by God and possibly the Christians. I thought Abraham was Jewish and all prophets were Jewish and God only spoke to the Jews. And, as I began to look around or reread texts (after being exposed to Islam...I have to admit

) it became apparent that I've been looking at it from a wrongly guided view. After learning that Abraham was from the land of Ur...I was in shock. So, anyway -a long story made short...
How did people of ancient times know about God?
I know that when Moses went into the desert he found a home with people (not sure if they were the Ishmaelites or Midinites or someone else) but, from what I understand they worshipped the one God. I don't believe there were any texts so, how did they know what to do? Did Abraham teach Ismael and Ismael teach his sons and so forth?
Thank you.
Another interesting topic and I was taught the same thing when I was young. I thought and believed that prophets only came through Jews but everything finally made sense to me after I read the Quran. As you said Abraham(as) was born in Iraq(Mesopotamia) and there was no such thing as Jews or Israelites at his time. It was through Isaac his second son that the Jewish prophets were born and Muhammad(pbuh) was a descendant of Abraham through Ismael(as). The Quran states that every community or nation in the ancient world received a prophet or messenger from among themselves to teach the the right path. Between the first Prophet and human, Adam(pbuh), and the last one, Muhammad(pbuh), God had sent thousands of prophets and 25 of them are mentioned in the Quran by name. If you think about it, that makes more sense because what about people in far off lands, what were they doing all the thousands of years?
We see today there are so many different ancient religions, and interestingly they almost all have something in common like a believe in one God, who is unique, etc. In Islam we are taught that over time, the messages of the prophets were forgotten, rejected, corrupted etc. The prophets were all sent with one universal and general faith which was submission to the will of God(Islam). Of course the laws differed from place to place and the language was different but the fundamentals were the same. There is only one God, who alone has the right to be worshiped, obey the prophets, etc.
As for Moses, he was an Israelite and the Torah was revealed to him by God for the guidance of his people. The Israelites were chosen by God and He had made a covenant with Abraham and his descendants that they obey him and worship none but Him. Of course those who obeyed Moses and the rest of the prophets who were sent were on the correct path and they regarded as Muslims(one who submits to God). the same goes for others as well. This does not mean that the Jews were the best and God only cares about them but He had given them a special test, He blessed them and made them powerful, yet they slipped away at times and disobeyed God. Some of them were arrogant and careless. They were ordered to obey the prophets but many of them were rebellious. They killed some of the prophets, rejected others, they even tried to kill the Messiah(Jesus) by having him crucified. Of course Islam differs with Christianity about the crucifixion of Jesus(as).
Since they rejected him and disobeyed God, they could not be favorite or "saved" as some say. Those who obeyed Jesus during his life and worshiped God were "saved" or on the correct path but that don't go for everyone who claims to be a Jew just as no Muslim will claim he or she is "saved" because only God really knows the future and what a person will do.
How did people in the ancient world know about God? First of all, we are all born with fitrah(pure nature and with the ability to know basic right from wrong and come to know the creator). We believe that every human is born in a state of submission to God, innocent, pure and with the ability to come to know our Creator. However, we are influenced by our parents, society, prejudices, desires, ice cream man, post man etc lol. When we grow up, if we are honest, open minded and put our family traditions aside and seek truth, we will recognize that truth when God brings it to us and we have the choice and ability to worship God or turn away and worship his creation instead which is wrong. For the people of ancient times, prophets were sent to every community of those days and those who obeyed them were on the right path while those who rejected were disbelievers.
No one was left in darkness and prophets/messengers came from among all nations and they all were sent someone so they could have true guidance. No one will be punished unjustly or for not knowing something. God finally send Muhammad(pbuh) as the final prophet and messenger with the Quran for all of mankind and for all times. He wasn't sent with a new religion but it was the same way of life only its complete and final version. He confirmed the prophets before him, testified of them and it is a requirement for Muslims to believe and honor all of God's prophets without exception.