There are a couple of major reasons to know church history. First of all, when we don't know church history we are bound and determined to repeat the same mistakes that were already dealt with. More dangerous than that is that we may bring back some of the same heresies. Another reason for church history is so that you do not argue against your history. Have you ever heard someone say that had never heard your view of something when you are closer to the overwhelming majority of church history?