You could also solve the problem in role-playing terms, like stating that religion's sacred armors negates any penalties with spell casting. Or you could give your custom made expensive armor to your religion's priest so that he can cast some purification/enhancement or so, that it could fully or partially dismiss any spell casting penalties. Just going and buying some pagan made armor from god-know what blackmarket place would give the penalties (or even more) written in SCM tables.
As for rangers, that somehow channel the nature's own magic, or power of creation (whatever it is in your campaign) without specific religion, this could mean that the armors made by ranger or his ilk dismiss these penalties. I guess there are some very hard trees out there that could, properly enchanted and crafted of course, provide AT 20 armors for example. It's a magical world, after all.
Just my two iron pieces.