In many ways I think it was inevitable that I would eventually get around to joining (the NSS) as their stated General Principles very accurately represent my outlook. I feel very strongly that religious beliefs are valid and appropriate when held and practiced privately, but that if you use your beliefs to justify your actions towards others then you must in turn be prepared to justify your beliefs.
What prompted me finally to join was simply that I felt I had had enough of religious organisations trying to establish themselves as being above criticism. That it is not possible to criticise or comment on religious practices without being told that you are being offensive frankly scares me.
My privately held views on dogma vs. reason are not something I feel I can act on publicly, but in Secularism there is a cause for which I can be politically active. Ultimately it's about removing the archaic rights given to religions to infringe my liberty.