What most makes a Christian Protestant is the issue of authority. Where do we look for authoritative truth to guide us as Christians? Protestants believe it is in the Bible. Sola Scriptura, the Bible alone, is the formal principle of the Reformation. I think that is crucial, especially in our day where there are so many doubts about the Bible. When one accepts the Bible’s authority, one will find that it clearly teaches the doctrines of grace alone and faith alone.