"It is either foolish or suicidal to die voluntarily unless there is some great good that can only or best be accomplished by voluntarily dying. . . . [So] Christ must have had a great good in mind that could only or best be accomplished by voluntarily dying [on the cross]. . . . The only great good that can justify a voluntary death is if that death saves other lives, and the only theory of atonement that makes sense of why the death of Jesus would save other lives is the theory of penal substitution. . . . Therefore, the doctrine of penal substitution is the only adequate explanation of Christ's voluntary death on the cross." - Steve L. Porter, "Dostoyevsky, Woody Allen, and the Doctrine of Penal Substitution", in Paul Copan and William Lane Craig, eds., Contending with Christianity's Critics (Broadman and Holman, 2009) 233-248: p. 244-45, 248.
If Porter is right here, then it follows that Christ must have thought that his death on the cross would act as a 'penal substitute' and thereby save us from physical and spiritual death. Put another way, Porter is implying here that Christ believed in the doctrine of penal substitution.
I have argued elsewhere that Porter's article gives us no reason to think that this doctrine is morally defensible. But since Porter's view is that penal substitution is the only 'great good' that could have justified Christ's voluntary death, it follows that he must have given up his life for no good reason.
Of course, Jesus would not have been the first person in history to have sacrificed his life in vain. Nor the last. There are countless men and women who have mistakenly believed that the only way someone's life could be saved would be if they sacrificed their own.
Some of these brave souls clearly should have given the matter a little more thought before they took the fatal plunge. We can often know, well in advance, that laying down our life will not save the life of another. Porter gives us a useful example:
"We think it either foolish or suicidal when a person jumps in front of a speeding train proclaiming love for a friend. Unless, of course, the friend (or someone else) is in front of the speeding train and jumping in front of the train was the only way to save that person." (p. 245)
But being foolish is not the same as making an honest mistake. It is more likely that Jesus was simply applying the (flawed) moral concepts and theological principles available to him at the time. In that case, he was just unlucky rather than foolish.
Porter, of course, thinks that Christ's decision to submit to death on the cross could not have been an act of foolishness; nor could it have been just an honest mistake on the part of Jesus. But both of these explanations can only be ruled out if the theory of penal substitution can be morally justified – which, as I (along with many others) have argued, it is not.