Robert Louis Wilken, in The Christians as the Romans Saw Them, had this to say when discussing why it is important to look at what outsiders were saying about Christianity in its formative years and not relying solely on Christian sources, as many are guilty of doing.
I am convinced that the perceptions of outsiders tell us something significant about the character of the Christian movement, and that without the views of those who made up the world in which Christianity grew to maturity, we will never understand what Christianity was or is. How something is perceived is an aspect of what it is. This is especially true in the social world, where the perception of others is an essential part of the reality people inhabit.
While we may not think everything said by an outsider is "true," it accurately represents their perception of the group, a perception which is certainly a part of reality. This truth has wide-ranging applications for Christians and non-Christians alike. Whether we want to agree or not, who people think we are says a lot about who we actually are.