Superman is far more likely a Methodist or another breed of Protestant Christian
This article goes a long a way to make the case that Superman is a Methodist, or at least some form of Protestant.
The article is a mess in terms of organization and readability but it does a great job citing comic examples of Superman's religious interactions. Some quick hits:
Action Comics #850 (August 2007), for example, identifies Methodism by name as the denomination that Clark Kent and his mother attended.
As shown in a number of published comics, including Superman: A Man For All Seasons, the adult Clark Kent continued to visit and consult with the minister at his family church, even after he had begun his career as Superman.
With the publication of Action Comics #850 in June 2007, the Methodist denominational affiliation of the Kent family was explicitly and overtly established, if it had not already been so.
Since the currently accepted answer cites Elliot S! Maggin, here's an undeniable quote from a Fanzing interview published in 1998:
I give all my characters religions. I think I always have. It's part of the backstory. It's part of the process of getting to know a character well enough to write about him or her. Jimmy Olson is Lutheran. Lois is Catholic. Perry is Baptist. Luthor is Jewish (though non-observant, thank heaven). Bruce and Batman are both Episcopalian and I said so in the text though it was edited out erroneously. Clark - like the Kents - is Methodist.
It's worth noting that over the years, Superman has sought advice from clergy of all sorts of different Christian denominations and has made several references to the Kryptonian religion, Raoism, which seems to me to draw inspiration from monotheistic Judeo-Christian religions and Ancient Egyptian religion. "Great Rao!"
A few comic panels of Supes interacting with religion (note how all of them are Christian in nature):
Clark and Lois were married in a Christian church by a Christian minister:
All of that to say there's no indication Superman was ever meant to be Jewish.