katholisch
Catholic
Relating to or belonging to the Roman Catholic Church.
In Bayern sind viele Menschen katholisch.
In Bavaria, many people are Catholic.

Catholic
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Catholic
Relating to or belonging to the Roman Catholic Church.
In Bayern sind viele Menschen katholisch.
In Bavaria, many people are Catholic.

As an adjective, 'katholisch' (Catholic) is written in lowercase when it describes a person or characteristic. When it is part of an official name, it is capitalized.
Meine Nachbarin ist katholisch.
My neighbor is Catholic.
The adjective 'katholisch' (Catholic) belongs to the word family around 'der Katholik' (male Catholic) and 'die Katholikin' (female Catholic). The ending '-isch' is a typical way to form adjectives from nouns that express belonging.
An diesem katholischen Feiertag haben die Geschäfte geschlossen.
On this Catholic holiday, the shops are closed.
Common combinations are 'die katholische Kirche' (the Catholic Church), 'der katholische Glaube' (the Catholic faith), or 'ein katholischer Feiertag' (a Catholic holiday). You can also be 'katholisch getauft' (baptized Catholic) or 'katholisch erzogen' (raised Catholic).
Ostern ist ein wichtiger katholischer Feiertag.
Easter is an important Catholic holiday.
Roman
'katholisch' and 'römisch' can both have to do with the Roman Catholic Church. In this religious context, 'katholisch' is often the right choice, for example for people, churches, or church services. 'römisch' is usually used in this sense in fixed expressions such as 'römisch-katholisch' (Roman Catholic). On its own, 'römisch' very often means something else: it can refer to ancient Rome or to the present-day city of Rome. That is why the words can be confused. As a simple guide: for the religion, you usually say 'katholisch'; 'römisch' needs a clear Rome or church context.

Meine Großmutter ist katholisch und geht sonntags oft in die Kirche.
My grandmother is Catholic and often goes to church on Sundays.
Im Text steht, dass der Orden zur römisch-katholischen Kirche gehört.
The text says that the order belongs to the Roman Catholic Church.