Religion is already a protected category. Arguably caste falls within both religion and race categories, but it is better to make it explicit until case law and state and federal legislation catch up.
In parts of the US, Catholics or Jews were not allowed to hold office well into the 1870s, so your example is not as hypothetical as you seem to think.
It does not matter what religion the discriminator is, it is the act of discrimination based on a protected characteristic (in this case religion) which is illegal. Imagine a Jewish boss rejecting Jewish candidates because he does not want his company to get the reputation of being only for Jews to apply to, that would still be illegal discrimination, even if they are of the exact same religion.
Before you accuse me of making up a contrived example, that was exactly why the New York Times downplayed the Holocaust during WWII despite by that time uncontrovertible evidence, because they did not want to be perceived as a Jewish news outlet engaging in special pleading:
> It does not matter what religion the discriminator is, it is the act of discrimination based on a protected characteristic (in this case religion) which is illegal
There is a nuance here. Sunni-shia is a sect rather than a religion.
There is a difference between:
1. A Muslim manager rejecting a Muslim candidate (without knowing or paying attention to their sect)
2. A Sunni manager rejecting Shia candidates while hiring Sunni candidates
1 is covered by current law while 2 is not (this is why Seattle passed this ban but for castes)
Your example won't apply here as the candidate won't be rejected due to being Muslim but rather being Sunni or Shia. If there were sects in play in the Judaism case, then that is an accurate parallel.
This is out of topic: Also, shouldn't laws go beyond just discrimination? Shouldn't we legislate towards prevent violence given the past and present of Sunni-shia violence? Discrimination in jobs seem tame compared to outright violence.
In parts of the US, Catholics or Jews were not allowed to hold office well into the 1870s, so your example is not as hypothetical as you seem to think.