When a non-Christian asks, “Is it fair for an innocent man to die for the guilty?” we naturally say, “No.” So how can we believe that Christ’s death on the cross justifies the sinner?
Before looking at the question itself, it would be good to consider the situation of being asked questions by a non-Christian. We need to be aware that sometimes we are asked questions like this by someone who only wants an argument or to justify their own unbelief. There are times when no answer is the best answer. Our Lord, for example, made no answer or comment to Herod (Lk. 23:9) and did not answer every question put to Him by Pilate (Jn. 19:9). However, we should always try to be ready to give a reasonable answer to an honest question, especially if it is a question we could have in our own mind.
In addition, we need to be aware that many kinds of “is it fair” type questions can be posed to us. This is often a way of blaming God for unrighteousness as a way of justifying disbelief. (Why should I believe in an evil God?) Discussions of such questions can be futile. So, unless you have a good knowledge of Scripture and a plan of response, it is good to avoid such arguments.
Let’s assume this is an honest question. In this case, the question arises from failing to distinguish different situations.
What is fair is really what is righteous. God is righteous and defines righteousness by His own character. The opening subject of the epistle to the Romans deals with God’s righteousness; and in particular, with His righteousness in accepting Christ’s sacrifice to clear guilt against mankind. The consequence is that He, the righteous judge, can account us as righteous based on our acceptance of Christ’s work on the cross for us. That is, God in righteousness accepts Christ’s substitutionary work on our behalf. If we accept Christ’s work on our behalf, we are reckoned righteous by God. The point is that human judgment is set aside. It is God who is righteous and must uphold His own righteousness.
Because the argument begins with human reckoning, “We … say”, and ends with what God declares, it implicitly assumes that the two standards, human reckoning and divine reckoning, must be the same. This is an “apples and oranges fallacy.”
Nevertheless, the question (or objection) raises some interesting issues. The fact is that human courts do, on occasion accept voluntary substitution; this sometimes happens in the case of monetary penalties. In the case of death penalties, that is an extreme penalty and is certainly a special case. We are in no position to judge the value of human life. How could we say that one man’s life is equivalent to that of another man’s life? The death penalty was instituted by God for the extreme crime of murder, and similar crimes. So, the argument for any similarity between the human penalty and Christ’s offering fails.
I think a final note may be helpful. We can entangle ourselves in many debates and sometimes these are unavoidable or may even be helpful. But, in general, a simple presentation of the gospel is what people need. Regardless of all the arguments a person may present to us, everyone has some sense of their own guilt. This is what the gospel is intended to satisfy. You may leave an argument feeling that you have failed to present a good case, but if you have been able layout a simple gospel message, by God’s grace that may in time bear fruit in the soul of the most hardened skeptic.