One possibility concerns a weakening of the association between the Republican party and Christian extremism in the minds of voters. To the degree that Democrats have been pushing Jesus too, it is possible that this association has become less salient for voters. That is, if both parties are constantly pushing religion, this ceases to be something to dislike about the Republicans.
Still, I suspect that the most likely explanation is that the religiously unaffiliated are growing increasingly dissatisfied with the Democratic party. Not only have they made a big deal of embracing religion, but the Obama administration and Democratic majority in Congress has not exactly brought us the sweeping change for which many of us had long hoped.