I can recall being at an age when I thought I pretty much knew it all. Not literally everything, of course, but certainly everything that was important. Most teenagers seem to go through such a phase, at a time when the knowledge they're soaking in is increasing at a faster pace than the size of their world. That phase hit me when I was about fourteen and peaked fairly quickly, but it took me a number of years to completely outgrow that arrogance.
It wasn't until my college years that I began to develop an understanding of the truth behind the somewhat cliche notion that the more you know, the more you realize how much you don't know. Even then, as my horizons finally began broadening sufficiently to narrow the gap with my book learning, it was a realization that I wore like a badge of honor more often than I allowed it to humble me. I still thought of what I didn't know largely in terms of book learning, being too inexperienced to fully appreciate the value of life experience, and still too full of myself to grasp the ramifications of the limits of human knowledge and the infinity that lies beyond them.
Unfortunately those who grow beyond that stage of intellectual and spiritual development are greatly outnumbered by those who never do, especially within the confines of organized religion. Within most strains of Christianity the Bible is viewed as an Answer book (with a capital A) that contains the sum total of everything important. Even the most rigid fundamentalist will acknowledge that there is knowledge outside the Bible (one can't turn to it to fix a line of HTML code, for instance), and even that there are things that are simply beyond human comprehension, but none of that is genuinely important. In their view the Bible contains a complete and unambiguous prescription for any life issue that a person could possibly face, and anyone who disagrees with their church's Answers or who finds an alternative ("unbiblical") solution that works better is deluded by Satan if not actively in collusion with the forces of evil.
Within most conservative evangelical circles, even those with an academic mindset have a strong tendency toward intellectual arrogance. Disciplines like science and philosophy are only valuable to the extent that they can be used to prove to the rest of the world how right we are. Every issue that's of any real consequence has already been resolved, and if only the rest of the world would acknowledge our God-given wisdom and authority its many problems would soon be a thing of the past.
Within such a mindset there is nothing of value to be learned from non-believers (including 'liberal' Christians), aside from thoroughly mundane matters and perhaps feedback on how to better refine our proselytizing techniques. All those who believe differently (give or take certain minor doctrinal points) are merely rebelling against God, and deep down inside they secretly agree with everything that we espouse.
Therein lies one more reason I was never willing to evangelize (at least beyond my high school years): sharing one's faith was all about being right and having all the answers - and I knew I didn't have all the answers. On top of that, I knew that a lot of the answers I did have were riddled with holes and assumptions. In recent years evangelism tactics have come to include acknowledgments that we don't have perfect answers for everything, but it's still very much about being right and about having the best arguments. It's still about telling, with listening serving only to pinpoint what to tell the other person. We patronize others with the best of intentions, but even the purest motivations make it no less belittling.
And how much do we really know, compared to infinity? The Bible has a wealth of knowledge and wisdom to offer, and to the Christian it's our greatest and most indispensible resource, but it's still a finite book written thousands of years ago in cultural and linguistic contexts that even the most educated scholars of our modern era struggle to relate to. If that fact alone doesn't humble us sufficiently to remove the condescension from our words, perhaps our world (and our perception of the God who created it) is still too small.
Twenty years from now I'll most likely look back with bemusement on how little I knew when I was in my 30s. At least, I hope that I never stop growing. God's creation is filled with wonder and I've only barely begun to scratch the surface.