It's hard to definitively choose one or the other as the oldest, as they were both essentially created at the same time. Christianity was born when people started following the teachings of Christ. One of these people was Peter. Apostle Peter (now St. Peter) was the first Pope of the Catholic church, so, in essence, Catholicism and Christianity started at the same time. Catholics were the first Christians; other denominations split later from the Catholic church when they wanted to live by different standards (think of King Henry wanting a divorce, then creating his own church when the Catholic church wouldn't allow it).
catholic was create first
