I've heard that in the Dark Ages Christians considered it a sin to bathe.
Why??

An interesting history of bathing prior to 1601:
http://www.gallowglass.org/jadwiga/herbs/baths.html

It does seem the early church condemned bathing because of the licentious behavior that occurred in bath houses, and those attitudes carried over to later eras. Bathing remained a public activity for many so the fear was that being naked in front of other people might lead to immoral conduct. However, according to this source at least, the idea that bathing per se was sinful is a myth.

When Christian Europeans first arrived in New England the natives were shocked at how badly they smelled. NAs in that area bathed daily and with some fussiness, while the Europeans perhaps bathed once a year. I don't mean 'at the bathhouse', I mean 'period'.

so was it just the Christians of the UK that didn't bathe or all of Europe?

Europeans were also quite shocked at how clean Japanese were - bathing, OMG, like twice a day!!!

Even desert peoples at least washed with sand :)

So when did they start batheing? After the black plague or did that not even make them more aware?