If many related religions such as protestant sects all worship the same God, then why have they separated and stay apart, not to mention religions with wider gulfs between them?
Because of differing ideas on how to worship.
Why would the same God want to be worshipped differently?
My God wants me to do this, but your God wants you to do that.
Therefore, I don’t think we are talking about the same God.
That would subject us to endless nittling sub divisions of a given deity. From an outsider’s perspective, we have to draw a line somewhere at what include as “the Same God”. Generally this is done through tracing the history of any given sect; if they worship the same being in name and mythological record, then it is the same deity regardless of worship practice.
Yep. Pretty much like we have today.
Methodists don’t go to Lutheran or Baptist services unless no other Methodist church is available. Perhaps it’s because of a nittling subdivision of some deity.
As I mentioned, I’ve met people in the same church who apparently have different ideas about what their God is and may “want” from them.
It seems much more likely to me that, whatever “God” might be, such differences in interpretation reflect the inadequacies of understanding on the human side, rather than actual differences on the God side.
Ahura Mazda and Ahriman, respectively (there are other versions of the names). Yes, that’s about what I recall from my brief world religions overview in high school.
But then, within a year after that, I met the one and only actual Zoroastrian (from India) that I’ve had as a personal friend, and actually discussed religion with.
I gathered that it was not so different from Christian notions of the struggle between God and Satan, wherein the match may seem roughly equal in many contexts, or even favoring Satan, but ultimately God was certain to prevail. The only clear doctrinal difference to me is that Ahriman did not in any sense come from Ahura Mazda.
And if their differences are great enough, they split apart.
I saw this happen in my community. One large Presbyterian church had a small group within it that didn’t like where the main church was going and how it was run, but weren’t influential enough to affect it. So they started their own church a few miles away doing it their way.
And while they might feel both worship the same God, they don’t worship in the same manner, and this was important enough to them to break strong ties with other worshipers.
“Once I saw this guy on a bridge about to jump. I said, “Don’t do it!”
He said, “Nobody loves me.”
I said, “God loves you. Do you believe in God?”
He said, “Yes.”
I said, “Are you a Christian or a Jew?”
He said, “A Christian.”
I said, “Me, too! Protestant or Catholic?”
He said, “Protestant.”
I said, “Me, too! What franchise?”
He said, “Baptist.”
I said, “Me, too! Northern Baptist or Southern Baptist?”
He said, “Northern Baptist.”
I said, “Me, too! Northern Conservative Baptist or Northern Liberal Baptist?”
He said, “Northern Conservative Baptist.”
I said, “Me, too! Northern Conservative Baptist Great Lakes Region, or Northern Conservative Baptist Eastern Region?” He said, “Northern Conservative Baptist Great Lakes Region.”
I said, “Me, too! Northern Conservative Baptist Great Lakes Region Council of 1879, or Northern Conservative Baptist Great Lakes Region Council of 1912?”
He said, “Northern Conservative Baptist Great Lakes Region Council of 1912.”
I said, “Die, heretic!” And I pushed him over.”
-Emo Philips
The ecumenical take is that God is mysterious, so nobody can be sure that they understand Him perfectly. So wildly different patterns of observance are to be expected.
I don’t beleive in any god (I start all my theological arguments that way to avoid confusion of why I seem to be arguing from multiple points)
Jesus’ teachings were specifically to bring gentiles into the grace of the god of Abraham. They may apply to the jews if a certain number of them wish it but the “old fashioned” way of attonement or The Covenant still works for them.
The dualistic idea, of two super-gods battling it out with each other aided by their cohorts of lesser-Gods, is definitely a Persian idea introduced into Judaism. Remember it was the Persians that freed the Jews from Babylon and "Israel"was part of the Persian empire.
After the Persian period Judaism develloped on it’s own again and became less dualistic and more monotheistic.
It’s the Christians that picked up on the idea again, when they started reading the Old-Testament texts. Giving more power to Satan by their interpretations of the texts, “untainted” by current Jewish doctrine
Maybe God doesn’t really care about the exact details. Really, I don’t think most monotheistic gods “need” much out of people. Religion is not really for God, but for humans. We take the basic idea of “let’s think about God” and make it more complicated for our own purposes.
I have friends who watch movies with me, and I have friends who go out dancing with me. Both are my friends, and I’m inviting both to my party.
Now, if you had friends that watched movies with you that insisted that they were your only real friends, while the friends that danced with you insisted that they were your only real friends, and you knew this but invited both to your party without correcting either one we’d have a real analogy.
And there’s those that order you to be their freind, and that it’s wrong to dance.
So…you invite them both to your party and hope everyone has a good time and sees that the “real friend” distinction is silly and not useful? I’m not seeing how inviting them both to the party isn’t “correcting” them both.
In the analogy, that is. I’ve lost track of what that means for talking about monothetic religions. 
Are there any truly monotheistic religions out there that started that way and stayed that way?
Depends on what you mean by “started that way”. Judaism, for instance, probably developed from polytheistic beliefs, but you could make a fair argument that it wasn’t Judaism until they went monotheistic. With Islam, the argument is possibly even stronger.
Maybe Ekhnaton’s, as mentioned, but it didn’t stay that long.
But is there, or was there a true monotheistic religion?
I mean no Satan no angels. no saints, no spirits, nothing. Just one God.
Islam.
As in, one single unitary being greater than humans? I’m pretty sure that you’ll find either “angels” or aspects, or both, in every religion, but I’d be interested in any examples anyone else comes up with. And I don’t think it’s even possible to have a religion without an analogue of saints: Any organized religion will have people who originally organized it, and those original organizers will surely be afforded special respect by their followers.
If there actually IS one God, then I think it’s fair to say that all monotheistic religions worship that one God, though their name for God may differ and their understanding of God’s rules may be more or less correct and complete.
It’s my personal belief that there is NO God, which leads me to conclude that all deists are worshiping fantasies of some kind. Since these fantasies are internally generated, though in most cases with “guidance” from spiritual and/or religious leaders, I think it’s obvious that there is not one fantasy they all worship. There are as many or more “gods” as there are brains of of people who believe in one or more gods. Even crazy Fred Phelps doesn’t worship the exact same God as the crazy members of his own congregation.