Indeed they were. And that makes perfect sense. If you grew up in smalltown America in the years the Kents grew up, you'd almost certainly have had at least a nominally Christian upbringing.

However, Siegel and Shuster had absolutely no interest in the Kents. They're both dead by the events of Action Comics #1, and are nothing more than a throwaway plot device to give Superman a bit of backstory, just as Krypton was.

Superman's roots were unimportant to Siegel and Shuster. All they were interested in doing was telling contemporary adventure stories featuring Superman.