Growing up, I often hear this phrase as though it was the advent of Hell itself.
One world religion. We had to do anything possible to make sure that never happened, which usually meant being very afraid of different religions that said the same thing or calls to unify religions.
Funny enough, I still find a one world religion terrifying. Not because of its bonding or unifying nature but because it's a religion. And religion is rarely about diversification, unification, the common bond of humanity or true experience of God. So, of course, one world of that would be hell. Maybe it is hell, I don't know. A religion that refuses to acknowledge the diversity of culture, of language, of perspective and experience. Yikes.
But, a one world spirituality... if there is a God that's the only thing that makes sense.