Answer:
It began when Jesus said to Peter that He was going to build His "Church upon this rock", (Peter means rock). Jesus then gave the keys of His Kingdom to Peter and told him whatever he (Peter) binds on earth will be bound in Heaven, and whatever he looses on earth will be loosed in Heaven. This meant that Peter had direct authority to create rules, standards, and guidelines to shepherd the new Church and feed Jesus' sheep. (Today's Catholics believe the apostolic descendent of Peter -- and therefore the Church's current shepherd -- is the Pope Benedict XVI, the Bishop of Rome.) The word "Catholic", which means "universal", had not yet been invented in the earliest days of Christianity, just as the word "Christian" had not yet been invented at the time Peter accepted his mission from Jesus. This new Church that Jesus built on Peter was both Catholic (because it was universally meant for all people) and Christian (because it followed and worshipped the Christ). Catholics are Catholic-Christians, as opposed to Protestant-Christians, who do not consider Peter as the original Shepherd of the Church.