[Tainnor]

Saying "catholics" instead of "Christians" is about the same as saying "Visigoths" instead of "Germanic tribes". I don't know why you would resist more precision. In the context of the times, it stands in opposition to e.g. "Eastern orthodox" (after the Great schism) or "Aryan".

[netdevnet]

I understand that it is a more precise term but my point is why does that additional precision matters in this particular context? All that is relevant is that a big external force came to unify a bunch of tribes. Christian seems good enough. On the other hand, why stop at "Visigoths"? You could list the actual sub-groups too. You can do the same with "Catholics" too.

But we don't because additional precision is not always necessary and in a context of several sub-groups, you would seem to emphasise a difference between them (by referring to the sub-group name) rather than their common name (Christians).

It's just something that I notice a lot in anglo-saxon (see non-Catholics) contexts and almost never outside the anglosphere.