The 19 most religious states -- ranked by Gallup as those who identify as "very religious" -- all went for former Massachusetts governor Mitt Romney in 2012. (Romney won 24 total states.) In those 19 states, President Obama averaged 39 percent of the vote. Here are those 19 states.
On the other end of the (religious) spectrum, the opposite is true. President Obama won the 14 least religious states in the country. He averaged 61 percent of the vote in those places; if you take out the District of Columbia, which Obama won with 91(!) percent, the president averaged 59 percent in the remaining 13 states. Here are those 13 states (and the District).
Looking back at the 2012 exit poll, you see a similar story. The more you went to church, the more likely you were to vote for Romney. Here's CNN's exit poll breakout on the question.
The predictive power of religiosity is nothing new. Going back to 2000 -- the first time they began asking the question on the exit polls -- the Republican nominee has won among those who attend church weekly by 20 points or more. The one exception is 2008 when John McCain beat Barack Obama among weekly churchgoers by 12 points. Check out this great chart from Pew that details the various factors that predicted the probability of an Obama vote.
Simply put: If you are attending religious services every week, you are very likely to vote for the Republican candidate for president in 2016. If you never go to any sort of religious service, you are going to be for the Democrat. It's a simple -- and remarkably accurate -- political predictor.