Basically what the title says. I guess a sort of muslim-christian demographic swap might be able to do this, but at the same time it's very unlikely that anything other than Arabic could be used as a language of Islam because a core tenet of the religion is that the Quran is the literal word of god, and therefore any language but Classical Arabic would have a hard time justifying it's use as language of Islam short of reformation of the religion. Christianity is a different problem as it's Liturgical languages are numerous and often local. Imposing a single language as the language of faith across most of Christendom requires the catholic church or an institution with as much sway as them.