In a currently running IMHO thread, The most significant person in the history of the whole world, sitchensis posts this:
Leaving aside the problem that “God” doesn’t qualify as a person, I have heard this from various numerous people of various religions throughout my life, both personally and through many media outlets. Where did this idea come from, and(if you believe it) how could it possibly be true?
If there is only onne God, then any religion which worships a True God ™ MUST be approaching that one deity, albeit those frog-faced heathens over there are doing it all wrong.
If you are talking specifically about Judaism, Christianity and Islam, they know it is the same god because their books show they have continuity from Abraham, so they all recognize Abraham’s God.
I’m not certain how to check up on this authoritatively, but I believe that all extant monotheistic religions are Abrahamic either directly, or by descent. So technically I believe that is a correct statement. The believers may quibble a bit about the etymology, or veracity of each “facet” of that deity, but is seems that the gods known as Yahweh, Jehovah, and Allah are the same being. It can be argued that the Abrahamic faiths borrowed heavily from the monotheistic Zoroastrianism which may include that defunct deity into that same group.
There are some other sects that may be monotheistic in practice, but since they are offshoots of extant pantheistic religions I’m not certain they qualify.
What about Sikhism, some sects of Hinduism, and (until fairly recently in history) the widespread worship of Shangdi?
Hmm.
Depends how we want to qualify the term Monotheism. I go for a strict interpretation myself, so According to the wiki on Shangdi :
And re: Sikhism:
I’m not knowledgeable enough about Hinduism to speak to that question, but something tells me that it won’t stand up to a strict interpretation of monotheism, probably being closer to Shangdi in practice.
Am I misunderstanding this remark? Zoroastrianism is not defunct.
I don’t even understand what it means to be “the same God”. It’s like saying “Imagine a pink horse. Are we both imagining the same pink horse?” The word ‘same’ doesn’t even make sense.
How does the Trinity and the Hierarchy of Saints stand up to a strict interpretation of monotheism?
I know persons who worship a singular & explicitly female Deity, so I’ll say no.
It makes sense to ask whether believers consider themselves to be worshiping the same God. Christians and Muslims pretty obviously consider themselves to be worshiping the same God as that of the Jews, for example, since their holy books (and history) make it explicit that their religions arose from Jewish roots.
It also occurs to me that most varieties of Christianity are only nominally monotheist. I don’t see a real difference between the ideas of Gabriel & Metatron (created ex nihilo by the head of the pantheonor transformed from mortal to immortal in the latter) and those of Baldr and Freyja (related to or conquered by the head of hte pantheon). In either case you have an immortal and supreme God (Yahweh, Odin) with lesser but still divine associates.
I know not a few Christians who consider that the Muslim Allah is not identical to their God.
And how do Jews feel about the Trinity?
Assuming you mean religious Jews rather then ethnic ones, I feel safe in saying Jews disbelieve in the Trinity. Of course, not all Christians believe in the Trinity either.
If we disqualify some sects because they were originally polytheistic, might we have to eliminate the Big Three? Some believe that Judaism was originally polytheistic.
I wouldn’t put it quite that way: more that Judaism evolved from polytheistic origins.
More goyisher nonsense. 
Well, there’s no god but Athena anyway. And Kara Thrace is her prophet.
I’ve been told by devout Hindus that they worship the same god as westerners. I think the motivation for this statement is clear and commendable - if my group actually worships the same god as your group, there is no need to fight about it.
But it doesn’t help, because the next step is that we all worship the same god, but you don’t understand him or what he wants, and we do. In some sense you might say that the God who had a son is different from the God who didn’t, or you could say that the belief he has a son is just wrong, though the god is the same. So I don’t think it holds up very well. It’s not clear to me that all branches of Christianity worship the same god.
They are simply uninformed. Mohammed identified his God as the God of Abraham, Moses and Jesus.
But would you consider the Trinity to be pretty much the same as the God of the Jews, just named differently?