That 1-in-5 American adults and 1-in-3 under the age of 30 claim no religious affiliation naturally leads to the question of whether this is a reflection of declining religious piety in the US or a product of social cocooning and a distaste for organized religion without any meaningful corresponding decline in the belief in an Abrahamic, monotheistic God. Even if it is the latter (and my best guess now is that the former provides the predominant explanation, something I didn't used to think to be the case), it's hard to imagine how stripping away the communal, social, repetitive aspect of expressed, organized religion and instead relegating it to the position of internal, individualized personal beliefs leads to anything other than a decline in the influence of its tenets and mores (ie, same-sex marriage*).
The following graph shows the percentages of all American adults and those under the age of 30 who attend religious services no more than once a year. Note this doesn't even allow for attendance on both Christmas and Easter, so these are effectively people with zero spiritual connection to any organized religion:
Coming of age in the mid-nineties, the generational cohort I'm a part of is the last one in which most members will have the shared experience of some sort of religious destination as part of their upbringings. In the next few years, those who attend religious services will be in the minority, as is already the case with young adults.
GSS variables used: ATTEND(0-2), YEAR, AGE(18-29)