.I also wouldn't agree with the premise that Superman ought to be Jewish because his creators were Jewish, because in the case of Superman the influence of their ethnicity on the character itself is negligible. Would Superman be the same person if he was created by say, Italian immigrants? He would be different, but not TOO different. Contrast that with, say, Will Eisner's THE SPIRIT, a series that is much more semitic in behavior, appearance, influence, and outlook.
Technically, he'd only be half Jewish. Martha Kent would be a Protestant. Clark would celebrate both Christmas and Hanukkah, as well as any Kryptonian holidays when he was older. I don't think Superman has to be Jewish just because Joe and Jerry were, I just think it would be something cool to add to the mythos. I could certainly tell some good stories with it.
I certainly wouldn't make Superman completely Semetic. Rather, I'd portray him mostly the same. As I've said, I'm not religious at all. I just enjoy the idea of coming up with religions for characters. It's fun.
Here's a question: what religion would the Varasto island natives practice?
Ah, that's one thing I've put much thought into. I see them practicing a form of ancestor worship, but many of them would also practice the religions of the unified Kryptonians. I'd see there being a big contingent of Helio Raoists there. BTW, let me explain some of the different religions:
Agnosticism - Obviously the name would be different, but it's basically the same thing. Jor-El would fall into this category, as would most of the scientists on Krypton.
Atheism - Belief in no gods. Again, same as here.
Helio Raoism - Worship of their sun as a god, identified with Rao (also the name of their sun). This religion was mainly used between Classical Raoism and Neo-Raoism, though there are still followers today.
Neo-Raoism - Worship of Rao as the one and only god, the being that created Krypton, the sun Rao, and the universe. This was Krypton's biggest religion before Machina Raoism. Lara would belong to this religion.
Classical Raoism - Worship of Rao and an entire pantheon of gods. This religion has declined, but still has a fair share of followers.
Machina Raoism - Basically the same as human deism. Believers see Rao as creating the universe and making it similar to a machine that runs by itself. They believe Rao has no further involvement with the universe.
Ancestor Worship - Similar to our own ancient forms, but Kryptonians have a better way to store knowledge, so it's bigger there.
Progenism - The worship of the god-like beings Kryp and Tonn, who created everything on Krypton.
Technoism - These Kryptonians believe that science should dictate every action Kryptonians make.