What does it mean to be Christian? If you got down to what it really means, cleared the slate and focused on your most essential doctrines, what are the important tenants of your faith? And who has the authority to decide such things for the church? These days, most of us don't think about those kinds of questions much. We might ponder aspects of it or debate the finer points, but by and large, the master principles were settled hundreds of years ago — precipitated by Martin Luther 500 years ago today.