After considering some of the replies in this thread, I have modified my definition of Christianity:
My original definition consisted of what I find the core belief of Chrisitanity to be, namely: The belief that through the actions of Jesus, mankind is (can be) reconciled to God. (Or, to put it in less neutral, more traditional words: Jesus provided the atonement for the sins of the world.) I find this to be the central tenet of Christianity because a) it is completely unique to Christianity and b) it gets to the heart of why Jesus (the central figure of Christianity) is followed, revered, and worshipped.
But I realized this definition is incomplete without including some aspect of what "following Jesus" would entail.
What constitues a "follower of Jesus"? Jesus made many requests and demands of his followers, but when pressed, there were two he claimed as most important: 1) Love God with all your heart and 2) Love your neighbor as yourself. So, to be a follower of Jesus, at the very least, one must attempt to follow those two commandments.
So, to combine the two: A Christian is someone who loves God with all his heart, loves his neighbor as himself, and believes that Jesus provided the means of reconciliation between God and man.