I don't even mind if they call their group a church. I just wish they'd be honest and remove the Christian association.
As far as the destruction of Christianity goes, as a nondenominational Christian, I think the reason for the current decline lies in the inevitable destructiveness that comes with institutionalizing around man-made doctrines and organizational constructs. I don't expect that many on this forum will agree with that, and that's fine. However, I think that we can all agree that many denominational churches are experiencing deterioration and the World is oozing in.
The good news is that, while religious institutions may tumble, the Church of Christ (sans "United") exists forever. Christ's Church is spiritual, and consists of all those He has added to it, from the beginning of time, by way of their faith in the promise and hope available only through God's only begotten Son, Jesus. Since the Church is not earthly by any means, it cannot be destroyed by those rebellious souls living here.
Remember also that New Testament Christianity had no viable presence from about 800 A.D through 1300 A.D. When the seeds of the renaissance were sown through the reformation movement it began to reemerge. And even then, it wasn't until the 1800s that New Testament Christianity really began recommencing.
Uh oh... now I've done it... let me don my helmet, secure my flak jacket, and brace myself for the onslaught!
I totally disagree with the premise of your fourth paragraph. However, I don't think this is really the thread to debate it on.
So, I will leave it alone and suffice to say that we are in substantial agreement that it is secularists (or whatever other term you prefer) from outside Christianity that have infiltrated Christianity and are seeking to destroy it from within.