First, practicing or not (where was that "fact" pulled from), they were Jewish and experienced their lives as Jews. The themes are obvious to those who actually know what a Jew is and what the Jewish experience was at that time.
The following year, Siegel re-used the name The Superman to develop a new character that became one of the most famous superheroes of all time. Shuster modelled the hero on Douglas Fairbanks Sr., and modelled his bespectacled alter ego, Clark Kent, on a combination of Harold Lloyd[5][20] and Shuster himself, with the name "Clark Kent" derived from movie stars Clark Gable and Kent Taylor.[9] Lois Lane was modeled on Joanne Carter, a model hired by Shuster. (She later married co-creator Jerry Siegel in 1948.)[9]
Siegel and Shuster's origins as children of Jewish immigrants is also thought to have influenced their work. Timothy Aaron Pevey argued that they crafted "an immigrant figure whose desire was to fit into American culture as an American", something which Pevey feels taps into an important aspect of American identity.[21]
https://en.wikipedia.org/wiki/Joe_Shuster
In his memoir, Siegel recalls how hed been increasingly frustrated with rising antisemitism in Europe and the U.S., and was very favorably impressed by a movie called the The Golem, about an avenging being who used his awesome strength to crush a tyrant and save those who were being oppressed.
Elliot S. Maggin was Supermans principal writer from the 1970s to the mid-1980s. His Judaism and studies of kabbalah and Martin Buber heavily influenced his work, and he acknowledged ascribing effectively Jewish doctrine and ritual to the Kryptonian tradition. That Superman is Jewish, he said, is so self-evident that it may as well be canon.
https://forward.com/culture/504342/superman-10-jewish-things-siegel-shuster-samson-moses-golem/