Eternal Septemberism is the belief that users are not wrong, rude, or stupid simply because they are new. Famous people are often critisized for claiming to be Eternal Septemberists in order to look cool. And that's about it.
As Eternal Septemberism is a state of mind and not a religion, it can be practiced in conjunction with Haruhiism. As other religions are less forgiving, they usually will not accept known Eternal Septemberists.
[raep] See also
Doctor Jay M. N. Squeeksand, a well-known Eternal Septemberist, and the creator of the Septemberist's Doctorate.