How to Explain the Necessity of Jesus' Death to Muslims

I have been in a few face-to-face debates with Muslims, and all of them were polite and respectful. One question from my Muslim friends kept popping up, however: "Why did Jesus have to die on the cross for our sins? If we sin, God just forgives! He does not need for someone to take our place." Muslims have told me that this substitutionary sacrifice is actually very unfair of God. God would never let a righteous man die for the guilty. They illustrate it this way: "Would God allow an innocent baby to die for the crimes of murderers? Of course not! So, it would be immoral for God to send a righteous man to pay for others' sins."

The Quran states very clearly that Jesus did not die on the cross: "They declared: 'We have put to death the Messiah, Jesus son of Mary, the apostle of God.' They did not kill him, nor did they crucify him, but they thought they did (Surah 4:157-158)." And it says that one person cannot bear the burden of another (Surah 6:164), so a human cannot die for the sins of others. (However, elsewhere in Surah 29:12-13 it says others, such as Christians and Jews, will be punished for the sins of Muslims. So, the Quran does teach that at least some will be punished in the place of others.)

To answer this I begin with the question, "What did the Messiah come to do?" Muslims call Jesus "Isa al-Masih" (Jesus the Messiah). So, what is it about the Messiah that is different from all other prophets? What is unique about His ministry? The Hebrew prophet Isaiah predicted the ministry of the Messiah 700 years before the birth of Jesus:

He is despised and rejected by men, a Man of sorrows and acquainted with grief, and we hid as it were our faces from Him; He was despised, and we did not esteem Him. Surely He has borne our griefs and carried our sorrows; yet we esteemed Him stricken, smitten by God, and afflicted. But He was wounded for our transgressions, He was bruised for our iniquities; the chastisement for our peace was upon Him, and by His stripes we are healed (Isaiah 53:3-5).

That was the ministry of the Messiah.

And then I go on to explain to my friends that we both believe that God is infinite, eternal, holy, and just (Psalm 90:2; Isaiah 6:3). We both believe that God's Law is perfect and anyone who breaks it must incur His wrath (Romans 1:18). The result of sin is death-- eternal separation from God (Romans 6:23; Matthew 25:46). But here is where the Muslim and I part ways theologically. The Bible tells us that no one is good enough to make up for our offenses against God (Romans 3:10-18, 23). The best we can offer Him is like filthy rags (Isaiah 64:6).

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