I experienced the same thing growing up a Baptist preacher's kid (in the Jerry Falwell, Jack Hyles orbit), but often with respect to groups of other Christians, not just individuals in our congregation.

What I mean is, the Christians I grew up with routinely defined other Christian groups as non-Christian.

Catholic? Absolutely, definitely not Christians. Same for other traditional apostolic churches like Angelicans and Eastern Orthodox. Categorically not Christians.

Methodists? Most of those backsliders are not Christians!

Lutheran? Why, most of THEM haven't been Christian since the days of Luther.

Pentacostal? Not just non-Christian, but heathen! Probably demonic.

Southern Baptist? Those liberals better get right with Jesus, because too many of them are not Christians!

Yeah, I know the Southern Baptist Convention is one of the most conservative groups of Protestant Christians today, but that was far from true in the '70s (How times have changed! I guess they got right with Jesus.)

Weird, huh?

It's all just weird.

It's an example of religion building theological sandcastles in the air and then using those fantasies to other and marginalize people with different fantasies.

To me, this is the primary evil of religion in general. Because religion is based on nothing objective or real, because it encourages irrational thinking, any sort of mistreatment becomes not only possible, but encouraged or desirable.