If you're too strict about sameness criteria, it's difficult to agree on what, like, pants are, and we wind up at the relatively trivial solipsism arguments that get boring pretty quick.
If you go back far enough, of course, the doctrines of the abrahamic religions _do_ agree on the fundamentals. "God made stuff, God is on our side, act accordingly." The procession of covenantal relationships throughout history (with noah, abraham, issac, ishmael, jacob, john, jesus, mohammed, etc. etc. ) continually reinterpret what that means for people, but the basics are something everyone can break bread over.
> ... fundamentals ...
If I may butt in. That's one such essential message of these religions, that we are all the same and equal. Yet their essence is to define the respects in which to compare for equality and in those they do differ substantially. The primary difference is the question of authority, which has differing answers IMHO, and I believe that's the same question as the GP was asking.
And so you pretty quickly end up with fundamentalism, except that they all cannot agree on whose fundamentalism, whose authority, whose definition of failure to accept.
Sure, but the criterion for sameness is what matters. I see this as cultural and historical sameness, not doctrinal; the doctrines of the Abrahamic religions certainly do not agree on the fundamentals.
I also don't view it as apophatic, as I acknowledge both positive and negative claims are possible. I just think it comes down to the criteria of "sameness" and so it's ultimately an ambiguous question, and one must specify what is effectively being asked. Depending on the actual question, both your answer and others can work.