The Atonement: Theory or Doctrine?

I'm currently reading through a short book on the atonement in the bible and church history. The author believes in penal substitution, and that it is one of many metaphors to explain the cross. The publishers' blurb on the back cover begins with this sentence:

The Penal Substitutionary theory of the atonement says that Jesus suffered the divine punishment for our sins at Calvary.
Early in the book Calvin is credited with being the first to give a "full-blown penal substitutionary account of the atonement" and "the first full statement of penal substitution," and again "the first complete statement of a penal substitutionary theory of the atonement" (emphasis mine).

In describing penal substitution the author at times designates it as a metaphor, at times as a doctrine, and at other times as a theory. Significantly, in commenting on Isaiah 53, the author says that this chapter "comes as close as anything in the Bible to teaching penal substitutionary atonement. Still, there is not a fully worked out theory here." Read the rest HERE