katolika

Katolika is the adjective; the person is katoliko and the body of belief is katolikismo. It is written lowercase like every other Esperanto adjective of its kind, and takes a capital only inside a proper name: la Katolika Eklezio.

The word-formation is worth a moment, because it does not do what the neighbouring vocabulary does. Kristano is built inside Esperanto: the root Krist- plus -an-, the suffix for a member or adherent, exactly as Eŭropo gives eŭropano. Katoliko is not built that way. It is a whole borrowed noun, katolik- plus the ending -o, and there is no root katol- for it to be a member of. Both patterns are normal Esperanto, and telling them apart is most of the skill; the machinery behind -an-, -ism- and -ist- for peoples, faiths and movements is set out in Landoj kaj popoloj.

Around it: katolikismo (Catholicism), katolikeco (the quality of being catholic, in either sense), romkatolika (Roman Catholic) when the distinction from other Catholic churches matters. Esperanto has a long-standing Catholic movement of its own - IKUE, founded in 1910 - which is one reason the vocabulary of the church is unusually well settled in the language.