You're not arguing that Cecil is religious. Your argument is that you projected religion onto Cecil. ...and that several other characters are more religious than Cecil. You also have this assertion that white magic is religious as, like, an inherent thing, and it's really not.
(Also, don't assume I'm some oldhead running off a bunk easy-mode version of FFIV; my first finished playthrough was the pixel remaster. You should play that, and you'll see that Cecil also isn't religious there.)
Again: Final Fantasy Paladin's roots come from a version of D&D from back when religion was just a small part of a much greater code of honor, and their 'fancier than just a warrior' magic happened to be holy-flavored to match. Look at Final Fantasy's knights and paladins across the entire series, and you see the same thing; Cecil, Steiner, Beatrix, both XI and XIV's paladins, even the Templar from Bravely Default, they're all tied up exclusively in the 'queen and country' knights, rather than the holy knights that followed after the D&D version they took their inspiration from, even if some of them have white magic (that, yes, does include the spell 'Holy' sometimes). Japan really doesn't care about Christianity except as flavor, and ESPECIALLY doesn't care about it in regards to paladins.
And I mention knights, because...
Yeah, there is a weird thing within the Final Fantasy series where technically 'Knight' and Paladin' are supposedly distinct jobs, they just happen to cross over in skillsets to the point where they usually just get blended together both in gameplay and depiction. Neither are ever religious!
Both are heavily-armored melee fighters, but the supposed notion that you generally see emerge early on is that Knight gets Cover and martial-styled moves, while Paladin gets white magic... but that's broken even in the first FF game. To give a quick lineup of what jobs have what:
1: Knight (with white magic)
3: Knight (with cover)
4: Paladin (with white magic and cover)
5: Knight (with cover)
9: Steiner is a Knight, Beatrix is a Paladin
11: 'Knight' in Japanese, 'Paladin' in English; Cover and white magic
12: Knight in Zodiac Age only, white magic, no cover.
14: Like 11, 'Knight' in Japanese, 'Paladin' in English; Cover and White Magic.
The only game other than IX that technically has both is Tactics; Knight is a base playable job, 'Holy Knight' is Agrias' custom job.
So yeah, the distinction is nonsense, but if you really want to split hairs, XIV's Paladin is actually a Knight according to the original writing.