I know, as a protestant, I underestimated this problem for a long time. I mostly talked about faith exclusively with reformed Christians. I had very little exposure to the wide range of opinion within Christianity. I have even less exposure to Christians of history. Modern mass media is starting to break this down but certainly when I graduated college 25 years ago I had almost no exposure to non-reformed ways of interpreting scripture.
The easiest answer to why others get it wrong is because they have made basic errors in faith or reason. Either they lack intelligence or they don't pray enough or they don't take the bible seriously or they follow traditions of men and not true Sola Scriptura. Essentially saying the problem is those guys are just not doing Christianity right.
That actually works until you get to know them. Once you interact with Christians who disagree with you then you have to recognize these assumption as not very charitable or you have to move into an even more hateful attitude towards them. Happily, in modern time, most people realize the hateful approach is not where God wants you to go. Unhappily hateful Christians are much more common on the net than they are in real life. Here is one example from a recent post:
I know that Bryan Cross has said that it's unkind to speak about him in the third person, and no doubt the words argumentum ad hominem will escape from his keyboard coming up here.At one time this argument was used to justify putting heretics to death. Now it is merely used to justify making personal insults. Jesus said if you call somebody a fool you have already committed murder in your heart.
But for you Reformed folks who are trying to figure him out, what I'm about to say may seem unkind precisely until the moment when one of your children, or one of your church members, or even a Westminster-trained pastor that you may know, becomes enamored with and traipses off to Rome. At that point, then, ask, what is the real unkindness?
The trouble is coming to grips with the concept that these people are not fools. They are not evil. They are not biblically ignorant. Yet they have not come to the ideas that I claim are clearly taught in scripture. Somebody is doing something very wrong. It becomes hard to escape that that somebody is me.
There is one more way out of this mess. You can blame it on God. This is the Calvinist idea that they are just not predestined to be saved so that is the problem. I shall write another post about that.