Throughout all of his incarnations, Superman has maintained his rural Midwestern Protestant upbringing, although rarely have the words "Protestant" or "Christian" been explicitly attached to his background.
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.
Instead of making his big trip to the fictional New York of Metropolis, he makes his way to Moscow to become not only the darling of the 1950s communist elite, but also the country's primary defence initiative...
One of his most recent publications is the novel KINGDOM COME (which is available through Warner Books) which came out in February 1998. It's part of the process of getting to know a character well enough to write about him or her. I do think Superman essentially adheres to a kind of interplanetary-oriented Kryptonian-based belief system centered on monotheistic philosophy, and I've got some ideas about it that I haven't yet articulated other than as backstory.Although possibly not "canonical" at the time that Maggin gave this interview, this notion appeared already to have widespread support and subsequently grew in popularity.Many writers and fans believed this denominational affiliation best captures and explains the character as he has been portrayed over the years.The father and super-powered son are framed in front of a Christian church (note the cross on the tower or steeple in the background).Later on this same page, Superman mentions "the solid, moral foundation my foster parents gave" him. Maggin, an observant Jew who is one of Superman's most popular and influential contemporary chroniclers, stated in a 1998 interview that Clark Kent and the entire family are Methodists.