All religions believe that their own religion is correct, and that they will be guaranteed paradise etc according to their religion, however if two people both live morally equal lives but believe in different religions, what justifies that one should be granted peace?
In the field of religious studies and philosophy of religion (which i have a fucking useless degree in), this is a very active question.
Broadly, there are three (maybe more) theories and views on the matter:
(1) Every religion is right and an
equally correct path to salvation. Basically, all religions have good in them and all aim to solve problems of humanity or the human psyche. They are all equally (this equally is important) as valid and useful for achieving salvation/liberation/the ultimate end goal.
(2) Each religion is right and is a
different path to salvation. This is often told in the form of the parable of the blind men and the elephant. Imagine ten blind men all feeling an elephant. One blind man grabs the trunk, he comes to believe an elephant is a long slender animal, like a snake. Another grabs the leg and thinks its all huge and stocky like a block. Another still feels the ears and thinks its like a large butterfly.
In this view, all religions hold a part of the truth, a unique part of the truth. Each lead to their own realisation of meaning or salvation according to their own systems. It's a matter of perception and culture, you can't really say either religion is better but it might not necessarily be true to say they are equal or the same.
(3) Only some (or one) religions are right and only some (or one) religion is the path to salvation/liberation/so on. This one is pretty self explanatory. Obviously, the above have some interesting philosophical pitfalls and implications. For instance, if you believe (1), all religions essentially do not need to be defined or differentiated, what makes them anything definable at all almost evaporates. On (2), is it really true to say all these religions do lead to salvation, even if the salvation are contradictory between them?
These things make 3 obviously attractive. Likewise, in that religion is a form of social cohesion and differentiation, it makes sense to create a "us" vs "them" dichotomy.