Why study Church history? Well, there are many reasons, but the fact that, as Michael Haykin says, it offers us a usable past is a very good one:
Here then is the key reason for the study of church history. The Christian past is indeed a "usable past" for it contains mentors for the living of the Christian life. It is vital to note that this is not hagiography, for undergirding the command to imitate past leaders and mentors is the object of those individiuals' faith: "Jesus Christ, the same yesterday and today and forever" (Hebrews 13:8). It is because Christ Jesus was at the very centre of their preaching and living that their lives can be imitated today, for the Christ never changes. Their Lord is our Lord. Hebrews 13:8 is not an ontological statement, as some have taken it, but an assertion that the faith of the preachers can be imitated since the one they proclaimed, and in whom they had put teir faith namely, Jesus Christ, was ever the same.
I just started Dr. Haykin's book. It's been a while since I read a church history book; I'm really looking forward to this one.