My answer is that religions differ essentially in terms of what they believe happens after death.
(By 'death' I mean biological death of the body.)
This can be understood as what different religions 'offer' to their adherents after death; or alternatively as what the adherents of religions want to happen to themselves after they die.
All religions agree that death is not the end - or need not be the end.
And there are a wide range of proposals about what happens after death.
What happens after death, including the factors that affect what happens after death, has implications for this mortal life - and this is why different religions end-up with recommending different behaviours and beliefs in this mortal life.
But the 'behavioural' differences between religions may be less obvious than the similarities between them - so the life of adherent of different religions may overlaps considerably, and even be superficially indistinguishable.
In summary, the behaviours of religious people of different religions may vary only quantitatively.
It is in terms of post-mortal destinations that religions show their qualitative differences; because if one thing happens after death then another thing cannot happen.
If you reincarnate, you cannot resurrect; if you become a spirit after dying you will not have a body; if your personality and self are lost then they are not retained; if you assimilate with deity then you do not live-with the deity etc.
This is what makes religions distinct. And this is why the commonly-expressed idea that ultimately, at root, 'all religions are one' - is completely false.
However, if death is regarded as the end of everything that is a person in life - if death is annihilation of body, spirit, soul and everything else - then that is Not a religion.
That is where the boundary between religion and not-religion should be drawn.
So, religions differ in what happens after death - but all religions believe that some-thing remains 'alive', persists after death of the body.
In essence: religions are systems of ideas relating to what happens to men after biological death.
But when an ideology believes that nothing of a person remains after death, then it is not a religion.
