It's really sort of a mixed bag for me. On the one hand, any sort of decline in Christianity is inherently a bad thing, but on the other hand, the decline of mainline Protestantism is to be expected (this has been happening for decades). I think that American Christianity has been more American than Christian for quite some time. I mean, the fact that so many people are more concerned with so-called "marriage equality" than religious freedom is telling.

So the main question is, what's the best scenario for the re-evangelization of America? Is it better to let Atheism take over, then swoop in once people have realized they made the wrong choice, or is it more important to let the majority of American Christians think of themselves as Christians despite their beliefs being closer to moral therapeutic Deism? Personally, I think it'd be healthier to start with square one, than to have to re-educate people who already think of themselves as Christian. It boils down to what's easier: converting a Pagan to Christianity, or convincing a heretical Christian that his views are contrary to orthodox Christianity?

Either way, Christianity has survived as a persecuted minority religion before, so it's safe to assume that it'll survive in the future as well.

P.S. I'm not necessarily saying that Christianity is currently being persecuted in the US (it'd be foolish to claim that it isn't happening elsewhere, given how many Christians have died at the hands of ISIS). If I were trying to say that Christians were currently being persecuted, I'd have said something like "Just like in the past, Christianity has survived persecution, thus it will endure this current persecution as well". Though, like many traditionalist religious people, I'm concerned that when it comes to gay rights versus religious rights, the religious are going to lose. You might say that's a good thing, but I clearly disagree. While you're probably going to think I'm wrong to be against gay marriage, I at least think I'm consistent in that I also criticize American Christianity for allowing no-fault divorce and contraception. I think it's silly that many conservatives are claiming to defend traditional marriage by opposing same-sex marriage, but are totally fine with divorce (which also goes against traditional marriage).

Concerning future persecution of Christianity, it only seems natural. As Western society continues to distance itself from its Christian roots, Christian values will become more out of sync with secular values. Eventually, the religious will be forced to suffer quietly, while "enlightened" Progressivists have their say as to the future of the human race. Have you heard of those Atheist folks that want to make passing your religion onto your children illegal? For many Atheists they dream of a future where religion is either non-existent, or neutered. Would they go so far as to ban religion? Probably not. But things like making proselytizing illegal aren't that far-fetched. Am I wrong to feel concerned when society is moving in a direction I think is wrong? Wouldn't you be concerned too if, say, America were instead in another religious revival?