Of Clark's parents, Martha is the more devout churchgoer. While growing up in Smallville, Kansas, Clark Kent attended Sunday church services at the local Methodist church with his mother, Martha Kent, every week until he was fourteen years old.
These aspects of the character are not speculative, but are canonical - established by in-continuity published DC Comics.
This does not mean, however, that the adult Superman attends weekly church services (he does not).
If asked if he is a Methodist, the adult Superman would not answer "no," but he would defer answering such a pointedly denominational question by suggesting that he respects people of all faiths and backgrounds and considers himself a servant of all humanity.
The character was created by Jerry Siegel and Joe Shuster [often mis-spelled "Joe Schuster"], both of whom were Jewish.
Superman is sometimes spoken of as being "Jewish." This may be an attempt to honor the fact that the writer and artist who created the character were Jewish.
The creation of Superman and his alter ego Clark Kent was a manifestation of the desire by Siegel and Shuster to "pass" in mainstream population and also to assert control in a world that had often left them feeling powerless, such as when Siegel's father was murdered.
As is often the case with a character or franchise of extraordinary longevity, Superman has been reconceived multiple times ("retconned" in comic book parlance).
It is over one hundred thousand words full of action, characterization, and plot sculpting. Instead of Superman's rocket ship crash landing in the wheat fields of Kansas, Superman: Red Son details his landing on a Soviet collective farm somewhere in Ukraine.
Instead of being , he is raised during the cold war with an appreciation for Karl Marx and a devotion to Comrade Stalin.