Are all religions fundamentally the same?

When you get something false (because it is false) asserted as often as that "all religions are fundamentally the same" - it is usually because of an unexamined, perhaps unnoticed, assumption. 

Disagreement arises among those who do not share this assumption - yet the root of the disagreement is too seldom unearthed in the heat of argument. 

I think that people started saying all religions were (at root) "the same" when people started focusing (almost exclusively) on this-worldly aspects of religion and began ignoring (or simply not thinking about) what happens to us after death. 

In essence: The disagreement over whether religions are "the same" hinges around whether we assume a religion is defined by what it says and does about this mortal human life on earth; or whether we assume a religion is rooted in what it says about the life of our soul/ spirit after we have died. 

For atheists (and therefore the mainstream of public discourse in the modern world), of course, death is The End; so it is unsurprising that their comparisons of religions focus entirely on the psychology and social behaviour of various kinds of religious people. 

Since there are many broad similarities in the behaviour of people of various religions, then - by these assumptions - it seems reasonable to assert that all religions are fundamentally the same, and that differences are superficial and contingent. 

The other group who assert religions are fundamentally the same are those Westerners who advocate The Perennial Philosophy: some "oneness" spirituality that draws upon mystical (often "Eastern") religions such as Buddhism, Hinduism, Sufism or the like; and upon the tradition of Neo-Platonism or Gnosticism (for example) either with a pagan or Christian vocabulary. 

Such people do believe in existence beyond death; but this existence is impersonal. Survival is without ego, without personality; perhaps pure passive contemplation, perhaps by assimilation into The One (from which all originated). Oneness spirituality regards this mortal life as secondary, temporary, superficial; indeed an illusion ("maya") - the only real-reality is in the permanence and changeless stillness of oneness.  

Therefore, from the Perennialist/ Oneness perspective; all specific religions are this-worldly, ego-focused, and delusional: therefore all religions are fundamentally The Same - i.e. wrong!

But if instead we assume (as I personally do) that the fundamental nature of a religion is defined by its account of what happens after this mortal life: that is after death - then we get a very different understanding. 

If the main thing about a religion is supposed to relate to this mortal life and world; then we may well conclude that the similarities are more important than the differences; and that all religions are - near enough - "the same". 

But if instead we focus on the nature of life after death - as I believe that we should - then we discover that there are many, qualitatively distinct and mutually incompatible differences between religions - and differences between any religion and other types of world-view and ideology. 

For instance; if we expect or desire to reincarnate after death there are several very different models of how reincarnation works (re-cycling of souls, spiritual evolution, operations of karma etc).

If one of these reincarnations happens to our soul, then the others don't happen! And therefore the religions are different.

Like wise with other possible outcomes. If like the ancient Greeks or Hebrews, every soul is assumed to become a demented ghost dwelling forever in an underworld; then this post-death outcome is absolutely distinct-from and incompatible-with  the Christian belief in resurrection of the body to eternal Heavenly life.  

In sum; all religions came be regarded as very broadly "the same" if we focus wholly on our bodies, on human psychology and sociology, during this mortal life on earth; but there are many distinct religions (and non-religions) if, instead, we define a religion primarily by its assertions concerning the nature or absence of existence of the soul or spirit after death.