Let's assume for the sake of discussion that God exists. What do I mean by "God"? For now, let's just say I mean what the average American means when they refer to "God". We'll circle back to that eventually. In the article I will be referring to the Abrahamic God with an upper "G" and other deities with lower cases "g's".
In the United States the predominant religion is Christianity in its many forms. Christians, as well as Muslims, claim to worship the same God as do Jews, who worshipped him first. Even people who aren't officially part of any of those three religions, who can in no way be considered religious, would default to the Abrahamic God when talking about "God". (We're specifically talking about the United States here, and even within the U.S. exceptions would include people who follow non-Abrahamic religions). Where did belief in the Abrahamic God originate? Is there anything to suggest that he really is the supreme creator of the universe? Or did he just have better press agents than all the other gods?
I doubt that there is any serious argument against the idea that every little tribe, every kingdom, every group of nomadic clansman had a god. I'm not going to argue about the probability that any of these gods existed in any real sense, but certainly people believed that they did. The people we know as the Hebrews and later the Israelites had a god as well. It's also certain that the Hebrews believed that there were other gods in addition to their God - just that their God was the best. Eventually the company line became that there was only one god...God...all the other "gods" were just demons, or were people worshipping Satan in a different guise, but it's indisputable that early on (it's in the Bible, people) worshippers of God acknowledged that there were other gods. If we acknowledge that early followers of God believed that the gods of other nations were just as real, it follows that if we believe that God is real, then those other gods must be real as well.
But what about the first few chapters of Genesis? Doesn't that say that God created the heavens and the Earth? Doesn't it say that he created the first man and woman and communicated with them? Yes, it certainly does. But every other tribe had a creation myth as well. Every other tribe had a story about how humans came to be. We still know what some of those were, and there are still people following the religions from whence those creation myths originated.
We've already, for the sake of discussion assumed that God is real. We're further assuming that other gods are likewise real. Let's extend that assumption and assume that God talked to someone and told them what how creation came about which eventually got written down in the Bible. Wouldn't it make sense that other gods talked to their worshippers and told them their version of the creation story? From an objective viewpoint we don't know which of these deities is really the creator. Or maybe the various creation myths aren't meant to be historical records, and were written down by people who liked to tell a good story. It's pretty well established that no creation story is physically possible. So, at best what's written down in the Bible about creation is God bragging about how great he is and other sacred traditions are other gods doing the same. We can extend this to the other early parts of the Bible as well. The "Law"? Sure, God setting down rules and regulations. Fine, other gods had their own rules, why not God? The books of "history"? Why not? Later writers assembling a legendary history of a people. Doesn't even need God to be involved. Psalms, Proverbs, Ecclesiastes, Song of Solomon? Some good poetry and lots of talk about how different human beings viewed God. The Prophets? People trying to make sense of current events. Again, not the only people who wrote books about legendary times or wrote down their opinions about what the gods thought about various issues.
Let's stop and take a breath here. We're still assuming that God exists. What we're not assuming is that anything that is written down about God is necessarily true. It might be, but it also might be just the opinion of one of God's followers. Or it might be the self-aggrandizing opinion of one god among many gods.
Around 2000 years ago a new sect of the followers of God sprung up. There had always been factions within the followers of God, who mostly were confined within the ranks of the people who were by that time known as Judeans (aka Jews). There were Pharisees and Sadducees, factions we know about from the Gospels, but also Essenes, Hellenists, and probably, like today, people who just got on with their lives and didn't give God much thought. This new faction centered around the figure that we know as Jesus of Nazareth. After Jesus' death and alleged resurrection, this faction, unlike previous iterations of Judean religion, aggressively proselytized, spreading their beliefs about God outside the bounds of their nation. At this time the various peoples within the Roman Empire still had their own national or tribal pantheons. But it was also a time when for various reasons people began to experiment with new and exotic religions from outside the borders of the Empire.
Let's stop and take another breath. Remember, we're still assuming that God exists, but we're not assuming that he is the only God around, or that the book purporting to describe his will for mankind is in any way superior to the many other "holy" books or mythologies. There's a running argument about whether Jesus existed. I tend to follow the logic espoused by Bart Ehrman, a professor of Biblical studies and author of many books on the subject. He views the four Gospels as historical documents. Not in the sense of "true", but in the sense that they claim to describe events that happened once upon a time. I've been listening to a podcast about the history of the Eastern Roman Empire. One of the subjects that continually comes up is the reliability of the sources. Sometimes there is only one source for a period of time. That source is then analyzed for biases, compared with what is known from other writers as well as internal consistency. Sometimes the only source was written decades or even centuries after the events it purports to describe. The Gospels in this respect are similar. The earliest one was written around 30 years after Jesus lived. Historians can examine each of the Gospels and study them in the same way that a historian would study an account of the Byzantine Emperor Basil II, looking for inconsistencies, points of agreement and many other points that I won't get into here. My point is that in addition to assuming that God exists, when it comes to Jesus, I'm reasonably sure that he existed as well, but with a lot more certainty.
So, we're now assuming that God exists, but only to the extent that he was one deity among many, but now we have Jesus, which we can assume with a lot more certainty existed. What does that tell us? Not as much as you might think. While we have historical documents attesting to Jesus' existence, and can reasonably conclude that a lot of what is recorded therein are things he actually said and did, we don't have any evidence that he was who he said he was or that his teachings really came from God. Which brings us to another issue. There are numerous contradictions within the Gospels regarding what Jesus taught and who he said he was. Each of the Gospel writers, not to mention Paul and the other writers of the New Testament, seemed to have different opinions about who Jesus was, what he taught and what the purpose of his death and resurrection was. Like the writers of the Old Testament, we can't be sure that the Gospels and Epistles weren't anything more than men's opinions about God.
As the years and decades and centuries ran on, a lot of arguments were made and even blood spilled attempting to determine precisely what the Bible actually said. The doctrine of the Trinity was the result of an attempt to reconcile the various Biblical views of who Jesus was, with opinions regarding why it had to be a certain way were tacked on every few years. Factions multiplied in Christianity's early days, shrunk as power was centralized and multiplied again at regular intervals. Today there are thousands of Christian sects and denominations, some differing from others in barely noticeable ways, others hardly recognizable to each other as having sprung from the same roots. In addition to the myriad institutional variations of Christianity, there are even more personal variations on who God and Jesus are, what prayer is, and what a Christian is; people whose image of God conforms to nothing in any creed or holy book.
And why should it? We act as if the Bible is an unassailable source of truth and that God's existence and his basic attributes are beyond argument. But if you're worshipping God, you're worshipping a tribal god who had a very good press agent and whose followers eventually pushed their beliefs outside the insular tribal ethno-state and out into the world. There's, of course, a lot of good in some versions of Christianity, and I believe that people who follow the more "love thy neighbor" strains of the faith are generally good people. But for myself I see no reason to ascribe to Christianity over that of any other religion, at least not on the basis that any of them have a lock on "The Truth"...or even "truth". I can be a bad person all on my own without justifying it with Bible verses and I can also "love my neighbor" without worshipping a Middle Eastern tribal deity.
We're responsible for our own actions.