Not a forum, but quite an interesting blog - and this article is 'relatively interesting' to this section. (See what I did there. )
http://www.relativelyinteresting.com/37-atheist-memes-arent-afraid-question-religion/
Perhaps the most striking trend in American religion in recent years has been the growing percentage of adults who do not identify with a religious group. And the vast majority of these religious “nones” (78%) say they were raised as a member of a particular religion before dropping their religious identity in adulthood.
As part of a survey connected in the PEW Research Center’s Religious Landscape Study, PEW surveyors asked these people to explain, in their own words, why they no longer identify with a religious group. This resulted in hundreds of different responses but many of them shared one of a few common themes.
http://www.relativelyinteresting.com/37-atheist-memes-arent-afraid-question-religion/