The Catholic Church makes a rather audacious truth claim that, to my knowledge, no other Christian sect makes. That is she claims to have the "fullness of truth." Quite the claim, and a rather foolish one at that is it not? The Church is, after all, a merely human institution, and humans being the fallible critters that they are cannot possibly hope to arrive at the "fullness of truth." I mean, we can probably arrive at some approximation of the truth. A set of beliefs that we hold to be true with the understanding that while they paint a pretty good picture of reality, there may be a few that deviate ever so slightly.
This was the case for just about every Protestant church I had been in. They laid out their beliefs, perhaps even ranked in order of importance or confidence, but there was always room left for error. We were told we had super high confidence in the basics, Jesus died on the cross for our sins, God exists as three persons in the Trinity, Heaven and Hell are real places, etc. And perhaps not as much confidence in the obscure things such as how old the earth is, what the order of the end times will be, is baptism by immersion or is sprinkling cool? To be sure everyone had their opinions on these less important issues, and it was these less important issue that made clear the distinctions between various denominations, but it was generally held that one can be Christian and disagree on such things.
The attitude was generally that of, "those other Christians are wrong, but our differences aren't too important so its best to live and let live." This is what I experienced growing up. Maybe your experience was different, but from what I've seen of Protestant writing those that aren't noisy attention whores tend to have some sort of "We can agree to disagree" attitude.
Why is that? Well because one of our premises in that first paragraph is wrong. The Church is not a merely human institution. Methodists have Wesley, Lutherans have Luther, Mormons have Smith, Catholics have Christ. Upon Peter's confession of faith Christ says:
Blessed are you Simon son of Jonah. For flesh and blood has not revealed this to you, but my Heavenly Father. And so I say to you, you are Peter, and upon this rock I will build my church, and the gates of the netherworld shall not prevail against against it. I will give you the keys to the kingdom of heaven. Whatever you bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven.
Here Christ establishes His Church by making Peter the first Pope. He gives Peter the power to "allow by an indisputable authority" and to "disallow by an indisputable authority" when he hands him the keys and furthermore promises that His Church will be preserved from the powers of hell. This is why the Catholic Church can say with a straight face that it has the "fullness of truth." Christ promised her he would preserve her!
So what do we have here? Protestants have room to disagree on virtually all things, whereas Catholics may not disagree with doctrines. Either because the Catholic faith truly has the fullness of truth, or because men have stepped in and imposed their will against God's. Either we're right or we're a cult.
Now, this leaves you only two options. Either the world's largest Christian denomination is a bunch of crazy and dangerous heretics, or Catholicism is the fullness of truth and you need to convert. There's no middle ground. There's no reconciling someone who is wrong when he believes that God Himself is preserving him! And there's no use fighting against an institution established and preserved by Christ! Either we're right or we're nuts!
Now people can, and have, and will, argue that atrocities and scandals that have afflicted the Church proves that this is all hogwash, but that's not what we're talking about. The Church is, as Pope Francis recently said, a field hospital for sinners. As such she is made up of sinners. She will always have such scandals (as will Protestants) but what the Church has not, cannot, and will not ever do is changer her doctrine. Its been clarified and codified through these last two millennia, but her beliefs have never changed.
So what's it going to be? Are we dangerous lunatics? Or are we right?