Yes, your agnostic friend is right, perfect justice would require that the sinner die. Yet perfect mercy would require that forgiveness be provided. Since Jesus took our sins upon himself, he bore the true guilt for our sin. Now he was the guilty one. Where the two curves (justice and mercy) intersect is Calvary. Does that work?
I heard a line of reasoning posed to me by an agnostic friend yesterday that I can't seem to answer. It started with the question, "If God makes all the rules, why did Jesus have to die? Why couldn't he just absolve human sin?" to which I replied, "Along with being perfect love, God is perfect justice. God's justice says we have all sinned and therefore must die; God's love desires to show us mercy and forgive our transgressions. These two natures of God are reconciled at the cross through Jesus when he took upon his body the righteous penalty for our sin." At this my friend asked, "But how can it be perfect justice if the one who sins is not the one who dies [as in Ezekiel]." I don't have a good answer and seem trapped. If I say, "God created the laws we have broken, therefore he can set the requirements for paying the penalty and grant us a pardon," then he can respond with a version of his first question all over again. If I respond with, "Justice has a righteous requirement that must be paid and Jesus paid it," then he will respond with, "But how is that 'perfect' justice, since the one who is owes the debt didn't pay it? If you commit a crime, I can't go to the judge and say, 'I'll do his time.' " How should I respond? (By the way, I told him I didn't know the answer but would get back to him and we placed a $10 bet that there was a good answer out there -- so I am really counting on you.) -- Jason Goble (Las Vegas)