Perhaps the modern day apostates don't say so, but the Baptists and Catholics of a Hundred years ago were pretty clear about the state of each other's salvation.
This brings me to another reason for not following any of them, their beliefs constantly change.
Simple lack of communication. Once they discussed their beliefs, they found the Great I Am was universal - except for the Muslims. They're still mad about Abraham giving away the goods they coveted. They've never gotten over that.