In 1 Corinthians 1:13 St. Paul asks, "Has Christ been divided?"
St. Paul's answer is 'No'. But ask yourself this: If Christ cannot be divided, then how can schism even be possible? Some Christians today, seeing all the various denominations, are tempted to think that the answer to St. Paul's question is "yes". Even some who recite in the Creed: "we believe in one, holy, catholic and apostolic Church" think either that Christ has been divided, or that the unity of the Church is only spiritual. If, however, the unity of the Church were only spiritual, then the Church would have no institutional unity; the "visible Church" would be reduced to the plurality of believers scattered across the globe. And if the visible Church were merely the plurality of all believers scattered across the globe, then there could be no such thing as schism in the visible Church, save apostasy, which, properly speaking, is not schism. Then since there could be no schism in the invisible Church, and since Christ's Church is constituted entirely of the Church visible and the Church invisible, it would follow that there could be no such thing as schism at all. But there is schism, and yet Christ cannot be divided.
How can that be? The fact that there can be schism shows that the visible Church is not merely the plurality of believers scattered across the globe. It shows that the visible Church is a body or institution. Given that the visible Church is a body (and not merely the plurality of all believers), and given that Christ cannot be divided, it follows not only that the visible Church is not purely homeomerous, but also that the visible Church has a visible principium unitatis, which cannot itself be divided. In other words, it follows that the visible Church has a pope.