Yes, I do think that Christians who are most knowledgeable about their faith are often the people who are most likely to confront Mormons, etc. about their faiths. Note: I said knowledgeable about their faiths, not knowledgeable about how to persuade other people or about how to treat other people with respect. The problem is that these people are clueless about how life works.
Consider the following analogy: Suppose there’s a Straight Doper who is educated in the ways of Cecil Adams. He’s memorized all his books, as well as the books of Jan Harold Brunvald. He’s kept up with all the current columns. He checks snopes.com every week to see if there’s any new urban legends being debunked there. He’s a veritable fountain of information about all sorts of general knowledge. Despite this, he’s terrible at explaining to people why the things they believe are wrong. He’s constantly implying that anyone who passes on urban legends is a complete idiot, scarcely deserving of his patronizing explanations. He makes sure that these people aren’t just informed of their mistakes but are humiliated. He has no idea when to walk away from someone who doesn’t want to be educated in the ways of Cecil Adams and insists on continuing all the arguments to the point of a fistfight. Such a person is a terrible representative of the Straight Dope faith, despite all their knowledge.
No, real Christians are not just people who “do good things” or who “believe in and recognize Christ”, certainly not if they also believe in the sort of heretical doctrines that Mormons believe in. You may not agree with this definition, and it’s certainly your right to call yourself a Christian if you feel like it, but that’s not the definition that most knowledgeable Christians accept. Right belief is not all of Christianity, but it’s part of it.
To use another Straight Dope analogy, the fact that someone sincerely and honestly believes in urban legends does not make them right. It does not give anyone the right to be obnoxious to them, but it also does not make them right. This is something I can’t say enough times: The fact that you are wrong about something doesn’t give anyone the right to act rudely to you, but it also doesn’t change the fact that you are wrong.
Yes, the most knowledgeable Christians think that Mormons, Jehovah’s Witnesses, Christian Scientists, and Unitarians are heretics, while Catholics and Eastern Orthodox and all varieties of Protestants are fellow Christian with some variant beliefs. The more knowledgeable they are, the less likely they are to be the sort that accuse members of other orthodox denominations of “not really being Christians”. Most of those who do make such accusations are not only not knowledgeable but are actually using denominational rivalries (and religious rivalries) as a substitute for what are really ethnic rivalries.
Asmodean, I hope you were joking in your post. If you were, go ahead and whoosh me. NOBODY IS SAYING THAT HERETICS DESERVE TO BURN. NOBODY IS SAYING THAT HERETICS DESERVE TO BE DISCRIMINATED AGAINST. NOBODY IS SAYING THAT HERETICS DESERVE TO BE TREATED RUDELY. I’m not even saying that heretics will burn in hell. I think it’s arrogant of anyone to claim to know what God will do with any particular person.
I sat next to a Mormon at work for over a year, and I got along with him. There are some Unitarians in a book group I’ve belonged to for years, and I get along with them. One Christmas vacation in grad school I stayed with an aunt who was a Jehovah’s Witness, and I got along with her. I would never start an argument about religion with any of them unless they initiated it.