In comparison to his older sister Susan(whose understated but deep piety has been a steady factor throughout the years, (see Infinity Crusade #1), it is owrth questioning just how devout Johnny Storm is in reality. We never even see him praying(unlike Sue) and in Fantastic Four 300# we see him and Alicia Masters- or the Skrull pretending to be her- being married by a clergyman of an unnamed denomination- but not presumably Catholic- in a secular chapel).

As with Sue, could we please have more stories focusing on this aspect of Johnny's life??/

Johnny isn't a character who is defined by his religion like Nightcrawler or Kitty Pryde is. He isn't really influenced on the comic book page by what religion he believes, he's more defined by his hot shot attitude and team funnyman with his hijiinks with Ben Grimm. A lot of characters have undetermined religious beliefs because that's not a main part of your character.