So if atheism was in fact driving this data, 2015 would be a low point for atheism, right? Where is your statistic for atheism in 2015?
Actually, there are a number of factors.
First, there might be a “lag time” factor where individuals lose their strongly held beliefs over time, while distinct changes in behaviour might take a number of years to manifest. Decadency might require time.
For example, Gallup has a definitive rise in atheism starting in 2011 and the NCVS a notable rise in violent crime beginning in 2015. If we allow a few years of lag time between loss of belief and definitive change of behaviour we might be looking at loss of religious belief being a driver for a trailing increase in violent crime. The next few years may be quite revealing.
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Second, mere polling on theistic/atheistic beliefs is not a reliable data set because merely declaring on a poll that one is a believer might indicate social or cultural affiliation but may not be truly reflective of the importance of religious beliefs (or lack thereof) in decision making or courses of behaviour.
To illustrate this point,
this chart is from Pew detailing the percentage from each religious/non-religious group that lack a belief in God or gods.
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So 2% of Catholics, 4% of Episcopalians, 3% of Anglicans, etc., were essentially atheists (lacked belief in God) in 2014 but identified as Catholic, Episcopalian, Anglican, etc. That would seem to indicate profession of atheism isn’t the same thing as behaviours consistent with atheism. Ergo a lag time between change of nominal affiliation and decline of moral beliefs/behaviours might take a few years.
If the rates of atheism keep climbing, as Gallup is finding, and increases in violent crime rate lag that by 3-5 years it will be interesting to track what happens in the next 10-20 years if atheism begins to prevail in the US.