Christians follow the teachings of Jesus passed down through his disciples. The most important of these disciples was St. Peter who is the founder of the Catholic Church. So, the Catholic Church is the direct continuation of the teachings of Jesus aka they are Christian. Fast forward past a few bad popes and you end up with the Protestant thought which you can categorize many of the modern Christian churches. In other words... BOTH CATHOLICS AND PROTESTANTS ARE CHRISTIAN! To be Christian simply means that you follow the teachings of Christ so any church that does that is technically a Christian church.