Problems with defining Penal Substitution
I have been musing recently on the question of the definition of "Penal Substitution". What are the key concepts that by being present make something "Penal Substitution" and by their absence make it not be that? There are two main dangers with definitions - too narrow and it applies to nothing, too wide and it applies to everything. Balancing these two and isolating a useful definition is often difficult.
In one recent popular work the definition of PS given is that Jesus "suffer[ed] instead of us the death, punishment and curse due to fallen humanity as the penalty for sin." That looks okay at first glance, but the more I look at it the more problems I am seeing with it.
The first two things I notice is that it says nothing about eternal salvation or faith. Now I suppose it's true that you could theoretically have penal substitution without those things, say where a court fined a man for stealing and his friend paid his fine for him. In such a circumstance there is substitution of punishment (though it's informal substitution, rather than court sanctioned substitution - does this matter?) but eternal salvation is not involved, and the man does not really require faith in his friend or belief or trust or assurance in the efficaciousness of substitutionary payment in order for the friend's payment be effective. However Penal Substitution as a doctrine involves faith and eternal salvation in virtually all explanations I've ever heard of it, so leaving those out of the definition perhaps isn't a good idea... without them something can be "penal" and "substitutionary" but not really "Penal Substitution" as we know it.
Next, I have an issue with "instead of us the death" in the definition. All Christians will die physically, just like any other humans. Christ died physically. He didn't die "instead of us", since that would imply he died and we don't. If something other than physical death is meant it needs to be specified - as it stands, the provided definition is false.
I also take issue with "death, punishment and curse". It's like they couldn't decide quite what they believed so they tossed a whole lot of stuff in. The ambiguity and unclarity about what precisely Christ is suffering on our behalf is unhelpful. Are we meant to take it that he suffers three quite separate things on our behalf - death, punishment and curse - are we punishmed for sinfulness three times over in different ways? Or is it meant he suffered the "punishment of death" or perhaps "a curse invoked as punishment"? Studying the ancient ideas about curses has made me realise that punishment and curses were pretty different to each other. Yet this definition stuffs them together as if they were the same thing, or implies that penal substitution can teach it is any of them (though it says 'and', not 'or'), or that people who hold to penal substitution are confused about which of them it is.
Also missing from this definition, it seems to me, is the idea that God as a source of the punishment(s) that Christ is rescuing us from. ie the idea of us being saved from God's wrath, or God's justice, or God's punishment is missing. It is just a punishment in general that we're described as saved from, not specifically God's punishment. As such, this definition is broad enough that the ancient "Ransom From Satan" model meets the definition. Whoops. I think that definitely shows the definition is too broad. Interestingly, the same book from which this faulty definition of PS comes identifies a passage in Eusebius that is teaching Ransom from Satan as teaching Penal Substitution.
I have been trying myself a few times to list a clear checklist of ideas which comprise penal substitution and run into difficulties. I've jotted down sixteen or so ideas that together comprise Penal Substitutionary Atonement and are necessary to differentiate it from Ransom from Satan and other views. I intended them to be one short phrase or sentence each, and most of them are. But two of the most important ideas on the list which are key in differentiating PS from other models, I have yet to work out how to express in under half a page. So I have so far not succeeded in achieving definitional perfection.
More and more I am coming to realize that there is a very important difference between day-to-day penal and substitutionary situations like my analogy of paying a friend's fine, compared to the Doctrine Of Penal Substitutionary Atonement Of Post-Reformation Christianity. I think that is highlighted in the Ransom from Satan comparison - things can include penal and substitutionary thinking and ideas without being Penal Substitution.