To me there is something to be said about tradition and history, and I for one do not believe that highland dress is a costume, it is a formal outfit, regardless of whatever Alice Cooper has to say about it.

That said...

Cirthalion asked whether or not he should wear the sgian dubh to church. If it is for your Christmas sermon, or another Christmas function, than I say go ahead. It is part of your heritage and it is traditionally worn as part of highland dress. If you have any doubts bring it up with the pastor and see what his take on the matter is.

If you dont want to wear a knife to church than I would say get a dummy sgian dubh, but it is an integral part of the outfit, so wear something.