One argument I’ve heard against the Catholic Church goes something like this:
Since the Pope cannot be deposed, and there is no person or group who can bound his authority, he is free to do whatever he likes, act tyrannically, and even unilaterally teach as divine truth whatever fits his fancy.
Protestants compare this with their own pastors, who are subject to a council of elders. These elders have the authority to elect or unelect a pastor–to vote him out essentially–a democratic notion that finds approval in our American minds.
So what about the pope? Does he have unilateral authority? Absolutely not! Paradoxically, the pope has less ability than most Protestant pastors to teach what he likes, for the simple reason that the pope is bound by the existing doctrines and Tradition of the Church.
The dogmas of the Church have, over the centuries, circumscribed a large circle around the theological and moral landscape of questions. He cannot move beyond the area enclosed. Instead, he is bound by it–and this is a good thing–being free to more deeply develop the understanding of the Faith within that circle, but not to break it.
Protestantism has no hard lines of dogma. The Oneness Pentecostals prove that even the doctrine of the Trinity is fair game for modification. As is baptism, the Lord’s Supper, moral issues, and many others. One Protestant apologist I have been dialoguing with elsewhere is known to maintain the belief that masturbation is acceptable. Sure, why not? His own opinion about what the Bible says (or does not say) is gospel as far as he’s concerned. The Catholic Church, on the other hand, cannot reverse her teachings like this.
So the pope, who seems to have all the authority in the world, is actually one of the most constrained men in the world. He is not free to make up or change the deposit of faith, but instead is bound to preserve it without corruption.