I think all the comments provide good examples. In my experience though, the most commonly used in this situation is also the simplest one: 言われている (or 思われている as well, like in the first comment that I initially somehow missed).
Checking on weblio for example, you can find the following example:
He's supposedly 85 years old: 彼は 85 歳だといわれている.
Now, obviously this is just one example since as many people commented there are other ways to express that. However, as I said in my experience this is very commonly used. After all, if you think about it, "it is supposed that" and "it is being said that" bear a similar, if not the same, meaning.
In this fashion your sentence could be translated as:
EDIT: I changed the word 舎利 I originally used for reliquaries with 聖遺物 as suggested by @Brandon in a comment. I agree that this is a more appropriate choice in this context..supposedly. :)