In Honors the other day, one of my fellow students remarked that back in Bede's time (we're reading The Ecclesiastical History of the English People) there weren't all the denominations we have today. You were Christian or you were pagan, that was it.
I respectfully disagreed. If you read Bede, you'll notice that he mentions the date of Easter roughly every other sentence. (Okay, so I exaggerate.) Almost the entire church in Ireland split off for a time because of two things: they celebrated Easter between the fourteenth and the twentieth day of the first full moon after the vernal equinox (rather than between the fifteenth and twenty-first days) and the priests cut their hair differently. (I kid you not. The way clergy cut their hair used to be a very important discipline.)
The Church has never, ever been perfectly united. At almost the same moment that Christ prayed "that they all may be one," Judas was getting his pieces of silver.
I believe that Christ founded His Church on Peter, otherwise I would not be Catholic. Thou art Peter and upon this rock... This rock. This fisherman from the hill country of a province of the Roman Empire, who spoke with a funny accent. The man who said that he would die with his Master and then denied Him three times.
This is our rock. The Catholic Church is Peter's church, with all its imperfections and failings and its scandalizing earthiness. It's a simple, concrete sort of religion, with stained glass and incense and statues in abundance, lest we all forget that we're in our church to think of God.
Yet it's Christ's church too. It's the church of Word-made-flesh, of a God who breathed and ate and drank and loved and suffered. It's the church of the one whose Cross was a scandal and a stumbling-block.
This is the Church against which the gates of Hell cannot prevail.