In the bible there are verses that talk about being on the mountain naked and praying to God. And that God replied to him / them giving guidance. No where in there did he damn them for being nude. Nudity was not the issue. The issue was not believing in him or following his teachings. It is mankind that forbids it for the most part. Because most people relate the simple act of being nude to sexual activities. Normal actives in life can be done nude and there is no sexual activity or sexual satisfaction involved in it. It is just normal activities But some can not see it that way. For them everything is sexual.
I think a better question is why is being nude so important? What is wrong with clothes? And if wearing clothes is a christian thing then why do non-christians wear them? Didn't many of the nations which were converted to christianity wear clothes before their conversion? Granted I don't think that many churches will accept nudists I don't think that many state institutions will accept nudists either(which is supposed to be separate from the church). Why is that?