Galant,

Basically, for a tartan to be "official" then it has to have the explicit approval of whatever the tartan is designed for.

My point was that if you were designing a tartan for all Christians to wear, you'd never be able to make it "official" because I doubt you'd be able to get a letter of approval from the "chief" so to speak.

Now you could conceivable get some particular church or denomination to officially adopt the tartan. Obviously this would involve some campaigning on your part, but it could be done.

Remember, though, a tartan doesn't have to be "official." Most of the tartans on record have never been officially approved by the clan, family, place or group they are named for and so are, strictly speaking, "unofficial" tartans. That doesn't stop people from wearing them, though. That's all I was getting at.
Aye,
Matt