It has become common in theological circles for historic doctrines related to the work of Christ to be described as "theories." Different aspects of the atonement have been commonly referred to in terms of models or theories for some time (e.g. penal substitution, Christus victor). Now, especially it seems since the release of Douglas Campbell's latest book, justification is being increasingly discussed in terms of "justification theory."
It's one thing for this to be the language of professional academic guilds, but I hope this language doesn't work it's way into the Church. Why? Well, I'm glad you asked. It's important because atonement and justification have to do with how we come into a right relationship with God. And from a pastoral perspective, I don't want to leave that up to theory.
Paul wrote of sinners that, "they are now justified by his grace as a gift, through the redemption that is in Christ Jesus, whom God put forward as a sacrifice of atonement by his blood, effective through faith" (Romans 3:24-25). And whatever that means, it is no mere theory. It is history; it is simply what happened. How do we gain access to God? Through the atoning work of Jesus Christ. How are we justified? By his grace as a gift. These are not theories for Paul. They are theological truths grounded in the historic event of Christ's death and resurrection. I don't want my relationship to God through Christ dependant on someones theory; I want it dependent on something that Jesus actually did.
The language of theory grants competing interpretations of atonement and justification some level of mutual credibility. The problem is that not all competing interpretations are credible. Not all are to be believed. Did Jesus propitiate the wrath of God or didn't he? Does God justify sinners or doesn't he? And the matter of whether and how he does that is not simply a matter of theory; it is an issue of what actually happened. It is a matter of what transpired on the cross, of what happens when a person believes the gospel. We must do the hard work of understanding what scripture means when it speaks of what Jesus actually did and what actually happens to us. Theories are of limited help; history is the key thing. There is a difference, and the difference may very well bear eternal significance.