"while the non-denominational Christian identity has been trending upward particularly since 2001."

I'm not entirely convinced that some of these non-denominational megachurchs are what they profess to be. I'm pretty sure that if you scratch the surface a little bit you'll find one of the mainline churchs just below the surface. But they are smart enough to figure out that they can attract more folks if they dress up the presentation a little bit. Of course - I could be wrong.