I wouldn't use the term fundamentalism as it encompasses things that are decidedly unchristian. I'd prefer to keep with the term Christian as opposed to hyphenated or pseudo-christian. Christianity is not a "just add philosophy" base mix kit for creating a religion. And unfortunately, most consider it thus.
Christianity at it's essential level is contained in the message of Christ - which message is contained in the 66 books of scripture. Christianity keeps to the warnings of Paul and the rest of the Apostles including Peter to turn away anyone who comes with a message that strays from the one they presented. This is the reason we have scripture to begin with. It is where it is enumerated for posterity exactly what was in their message. Paul, if you'll remember, actually gave a list of the things he'd preached when he told people to keep to his teachings, lest anyone be unclear as to what it was he was referring to. And it so happens he covered all the things he enumerated in his writings - leaving no one to guess what it was he said.
You see, they were dealing with errors creeping in. And lest someone think them so uncrafty as to not prepare for it, they beat everyone of the detractors to the punch - undercutting the possibility of error creeping in by defining what was not error and saying keep away from all else. Christianity sticks to the core teachings as directed. There is nothing more or less to it than that. And if you need examples you've already seen one. Christians don't pray to the dead seeking favors (including prayer). They recognize it both as sin and as being ineffectual. Consider the fool who knows escaping jail is against the law and will get him a stiffer penalty but who insists on attempting to do so by whittling away at a 10 foot thick cement wall with a toothpick and gets caught. I add in the last part because where sin is concerned, you're caught as soon as your heart concieved to do it. Not by man; but, by God who will judge you for it. So the guy gets caught and he is guilty of rebellion and stupidity. Where does that leave the cellmate who's been trying to tell him all along that he shouldn't be doing that and that it simultaneously doesn't work? Just a rhetorical question for you to ponder. But this is the work of a Christian, to point out errors for those who presumabley want to do the right thing.
Christians also know the ten commandments. You will not make statues to yourselves of anything in creation or in heaven. You will not bow to them, worship(venerate) them, serve them, ect. Other sects see fit to utterly throw this out the window and rewrite it with reason to say you can do all of the above as long as you don't make a blood sacrifice and as long as you call it "veneration" instead of "worship". The words are 100% synonymous, it's just that most don't know that to be so.
The root of many of the problems can be summed up in a single word - Philosophy. Philosophy is a tool of reason. And through philosophy, the hyphenated preteded Christians think they can construct anything they wish and tack it to Christianity if it sounds good to them. The mistake in that is multiple in scope. First and foremost is the mistake in authority. Man did not author the covenant and has no authority to alter it. It was authored by God and given as a take it or leave it prospect. IE when God finished speaking on the matter of what the Covenant entailed, it was done. He sealed it with the Blood of his son. The only P.s. was added By God and it was "here is the Holy Spirit to help you achieve what I mandated."
The second error in Philosophy wonks is this - you cannot serve two masters. You can serve God or you can serve your Philosophy; but, you cannot serve Both. This was the thrust of the statement made by Christ with regard to the rich man and his money - remember 'you cannot serve God and mammon' 'you cannot serve two masters'. Paul and Peter understood and spoke to this. Philosophy, btw, is where all the stuff about Mary comes from, assumption, perpetual virginity and all that nonsense. It is nowhere in the covenant or in history. It's unfounded philosophy constructed on faulty logic and bad reasoning.
Christians understand that the foundation of the church - our individual foundation - must be free from error. Thus when it comes to questions of whether or not something is true, Scripture is the rule - not hearsay or opinion. The motive behind hearsay and opinion is not known; but, the motive behind scripture is clear. It is the record of God's message and can be relied upon and tested. The hyphenated sects as a rule, prefer their opinions, hearsay and philosophy over the statements of the Apostles and of Christ. Christians understand this to be utter and complete error.
Any further questions? Anything unclear? Anything at all. I really want to give you a good picture; And I'm being as frank as possible so that I am in no way unclear. Just let me know.