A lot of people I hang around with seem to think "Christianity" is a single thing, rather than 45,000 different denominations with wildly different beliefs.
They in their minds when they refer to this single thing I think they're equating Christianity with Southern Baptists in the US. If one sect gets to be the dominant default, it should be Catholicism (~1/2 the global christians with 1.4 billion people, and ~5x as many in the US as Southern Baptists), but I think probably we should just remember that there has never been "one single" Christianity.