A younger, far more idealistic version of myself would say that this shouldn't matter, but realistically I think the only way it doesn't is if both parties are generally disengaged from religion.

I've learned through experience that even latent differences can be an indication of overwhelming differences of opinion on a variety of other, more crucial relationship issues including: sexuality, aspirations (living life in terms of this one or in terms of an assumed afterlife), gender role expectations, thirst for knowledge (or the sources that knowledge comes from), empathy, motivation, etc. Perhaps this is all more of an indication of how our past and upbringing affects us in rather insidious ways than anything else, but religion is forever bound to these aspects of our lives..

Of course it can all be overcome, and the degree of difference in devotion plays a big role in how much of a struggle it is to do so. I've seen it work, and I've witnessed its failure (many examples of both these cases actually since I was raised Unitarian and a majority of the people at that church were living this precise scenario).

Anyhow, my hopelessly rational approach to dating dictates that it's vastly more practical and ensures less drama and lots more enjoyment if I avoid the issue entirely by dating people in the range of agnostic to atheist.