It’s a very, very old term. Batman did not come up with it. It was the preferred term that the Reformers used when taking about the entire church - so as not to exclude those in Rome, or Constantinople, or anywhere else (and I’m sure it was used well before then). Lutherans, at least, have always recognized that the church exists wherever the Word is preached and Sacraments rightly administered (even if we think the Truth is less clear in a different communion).
In my mind, this is why it would be a severe blow to the faith if Christendom lost its largest and wealthiest component. Then again, that might only serve to strengthen the ecumenical ties between other groups - Christianity has a habit of doing well under persecution (the Gospel tends to have that ‘hopeful’ effect!

). Perhaps Christians would, necessarily, band together, as they already do in areas of the globe that see those of the Faith persecuted. But the fact remains; 1.2 billion less Christians is a hypothetical I’d rather not imagine.
God bless Memaw. And everyone else, too.