Catholic is slightly confusing as different Christians use the term differently. Catholic means universal. The Roman Catholic Church calls itself the, “Catholic Church.” Roman Catholics believe that they are the one and only universal church. Non Catholics are seen as heretics and are not necessarily considered part of the universal Christian church.
Christians outside the Roman Catholic Church of course consider themselves part of Christendom. The Anglican Creed includes,”I believe in the Holy Catholic Church”. For Anglicans the universal church includes their church and is not exclusively Roman Catholic. Lutherans and some other Protestants talk about themselves as part of a catholic or universal church.
There is a good and a a bad side to Christianity, see the category page