It mostly has to do with remebering things about your religion. Wisdom i can maybe see but charisma is defintely not - and the checks are the same across all classes too which is another important thing
Remembering things is the primary purpose of the check, but sometimes how you're applying it might require an alternative skill check to make sense. Here are some examples (from https://rpgbot.net/dnd5/characters/impractical-skill-checks/ ):
Wisdom – Consoling and counselling a creature through a crisis of faith could be done with a Wisdom (Religion) check.
Charisma – Proselytizing for a faith makes more sense as a Charisma (Religion) check than a Charisma (Persuasion) check.
I think the second example would do better as Charisma (Persuasion), with a situational advantage for it being specifically preaching