Catholicism is the faith and the whole way of the Roman Catholic Church — the church that has the Pope in Rome. It is a noun: the religion, the teaching, the Mass, the priests, the long history. A Catholic person belongs to that church; Catholic with a capital C is also the adjective. Though actually there is a quieter older word, catholic with a small c, which just meant “wide, for everyone”; you still meet it in careful writing, but daily talk almost always means the Roman church. In English-speaking countries Catholicism names one large Christian family, beside Protestant churches and the Orthodox churches. The word is formal: history class, news about the Pope, talk of schools and holidays. It is not a slang word and not a joke word, unless someone is being unkind on purpose.