When it comes to Jesus it may be alot more necessary to believe He was a historical figure. In fact you'd be hard pressed to find someone who doesnt believe He was historical. It would be impossible to explain many things about our history without a man named Jesus who died and rose again. If he did not live He wouldn't have had followers, if He did not rise again they wouldn't have kept following (no one would follow a dead Messiah... read the Maccabees). But it's not my belief that he came along time ago that makes me His follower, it's the fact that I believe he is here now and calls us to follow Him now. Someone can actually follow Jesus without a history lesson.
For me it is also important that Jesus came into history because I believe the Kingdom of God is here. I believe that the Kingdom of God is historical reality either now and/or in the future. Jesus coming into history helps make sense of that. Heaven is not some transcendant "pie-in-the-dky." It is here for us and it will continue to change the world within history.
when it comes to what is allegory and what is literal I don't get to draw the line, it seems to me that the Bible does. Scripture, if we know it well enough, will reveal what is truly important within itself. You must always ask yourself what the text is really trying to say. And we must alway treat the text as it is and not make it whatever we want it to be. You'll find in reading my blog I usually interperet scripture historically. I always ask the question, what did this mean to the writer in his place in history? Because we cannot argue that the writer didn't live, he must have, he wrote. I also ask what did it mean (in case of a story) for the character (Jesus, Paul, Moses, etc.) in his/her place in history. becuase the writer must have written with that question in mind (weather the story actually happened or not). What cannot be argues is that the text itself is historical, it was written by a real person in real history. And we must always examine the text consiouely.
Some books to spark your interest on these issues: Adventures in Missing the Point by McLaren and Campolo, The Challenge of Jesus by N.T. Wright, Christian College Christian Calling Particularly Keith Reeves’ chapter, The meaning of Jesus by Marcus Borg and N.T. Wright, Velvet Elvis by Rob Bell (AMAZING BOOK), why not The Divine Conspiracy by Dallas Willard, and the Historical figure of Jesus by E.P. Saunders.