One of the most common and unfortunate theological mistakes Christians make is assuming that every aspect of their faith is equally important.
One need not look far for examples: a simple Google search for such topics as "creation," "eschatology," or sadly, even "Republican," will turn up countless websites run by sincere believers in Christ who equate their certainty regarding some particular aspect of some debated issue with the very foundation of Christian belief.
For many, the specific nuances of their way of interpreting Scripture (usually in accord with whatever tradition they hailed from) are essential issues. Each one is every bit as important as all the rest, and if you throw out even one, then you might as well forget the whole thing.
The unfortunate and dangerous consequence of such thinking is threefold: it creates needless and counter-productive division within the church, it weakens the message of Christ to those outside the church, and it makes for a very unstable foundation on which to base one's individual faith.
Regarding the first point: Jesus intended His church to be unified.1 This of course does not mean that we should all worship the same way, or even that we should all share exactly the same beliefs (indeed, I tend to think that some division is actually a good thing - see here). It is, rather, much deeper and more significant than this. The context of the cited passage makes clear that the purpose of the unity is so that the world may know who Jesus is and that God loves them just as much as His own son. And we know from elsewhere that the way the world is to identify followers of Christ is by how they love one another. So it seems that what Jesus meant by unity has much more to do with how we treat one another than with whether or not we agree on everything. But when we assert that every part of our faith is just as important as every other part, we are in effect denying that loving one another is more important than being right about x, y, z. We are saying that Jesus was, well, wrong.
To illustrate the second and third points, a not-so-hard-to-imagine, imaginative scenario:
Susan shares her faith with Bill. Susan is sincere and convincing, and Bill can tell that there is a peace about her that he does not possess. From their conversation, Bill begins to consider Christ in light of the love he saw in Susan, and he comes close to choosing to follow Him. Then Bill meets Dan. Dan attends a particular church in town, where they are certain that their brand of Christianity is the right one, and that it is wrong to begin to question even the smallest part of their theology. Dan invites Bill to church. He goes, and is promptly warned that Susan's brand of Christianity is false and dangerous, because she does not believe the same thing Dan believes about some issue. It is obvious, Dan says, to anyone who really knows Jesus, and besides, it says it right there in the Bible, in plain English. Bill is confused, but he does not know how to respond to Dan. Both of these people claim to follow Jesus, he thinks, but there is no sense in which they are unified. He does not see how his opinion on issue x will give him the peace he thought he saw in Susan, and he leaves the church, frustrated with Christians and their division.
Or an alternate ending:
Bill takes Dan's word for it, and joins his church, accepting that each link in the theological chain is just as important as every other. He then reads a book (or hears a message, or takes a class, etc.), in which one of his church's beliefs is cleanly refuted. This one domino knocked over, his whole faith crumbles, since he has given equal weight to each piece.
The first scenario shows how our insistence on the ultimate importance of every part of our faith weakens it to the point of ineffectiveness in evangelism. The second shows how it weakens it to the point of ineffectiveness in maintaining one's own faith.
This, however, is not the biblical model of faith, nor is it the one with which the church has operated from the beginning.2 That model, rather, is something more like what Greg Boyd has called a "concentric circle" model of faith. The idea is simply that at the core of Christian faith are the essentials - the most important things about being a Christian, what the reformers called the sine qua non of the faith. Built on top of these things are the less important things, and the further out you go, the less important the issues become.
In the very center circle would be what a friend of mine calls the "Big, Big Question." Who was Jesus? It is Him, and Him alone, that grounds Christian belief and practice. If He is who He claimed to be and what He taught is true, then we have hope. If not, then we are "of all men most pitiable."3 It is this circle and this circle alone that decides one's eternal fate. This is the hill we die on.
Just outside this would be what we would consider the essentials of orthodox Christianity. These are the things that define what it is to be a "Christian," the things that were set forth in the creeds back in the day, the things that all Christians even today agree on (believe it or not, there are several such things). These are things like God's triune nature, the dual nature of Christ (fully God, fully man), etc. To disagree with these is not to be outside the scope of salvation (we cannot know this), but it is to be something other than historically Christian.
A little further outside this ring are doctrines more specific to particular traditions; outside that are issues that are important but not rigidly defined by any tradition; further out still are things we just like to argue about but aren't really important at all, like what month Jesus was born.
As my pastor puts it, "major on the majors; minor on the minors." Not everything in the Bible is equally clear. Not everything we believe is equally important. Let's try to do a little better job of loving each other, and perhaps focus a bit less on who is the most right about the gray areas.
2 For a brief overview of what exactly Christians mean by "faith," a word generally synonymous with "trust," see here and here.