What to know Your friend may have been told that Christians use the cross as an excuse to live in sin. This statement is often used to convey disgust at this (false) assumption. See Islamic apologist Zakir Naik's discussion of this idea. Your friend might also be conveying the Islamic concept of piety: That it's more reverent to not accept the idea of substitution. First, ask questions to understand which of the two your friend is saying. (Or if they have beliefs and questions outside of what we're outlining above.) Listen closely. Then, look to the Word of God. What to say
If your friend is responding to the idea of Christians using the cross as an excuse to sin:
Read Romans 6 together! Discuss.
Read Ephesians 2:8-10 and 1 Corinthians 1:28-30 together. Examine the question: How does "salvation through works" lead to boasting? Why does God oppose this? What motive does God want us to have for the "good works" that we do? See 1 Corinthians 13 to examine more clearly.
If your friend believes that substitution is not a theology from God:
Read the stories of the prophets that came before the Messiah. These revelations were sent to help us understand that substitution has always been his desire for us. 1. Tell the story of Nuh (Noah). God has the authority to pour out wrath on mankind…but he also had the authority to save. Notice that when God provided a way to survive the flood, Nuh had to respond in humility and in faith, and enter the boat. If Nuh had tried to climb the highest mountain, or swim through the waves, he would be rejecting God's provision - and he would have drowned. God is asking us to respond like Nuh, and walk in faith. Consider closing with 2 Peter 3:6-9 and Acts 3:19. 2. Tell the story of Ibrahim (Abraham). Ibrahim was told to sacrifice his son...And then God provided the sacrifice. God has the authority to ask for sacrifice, and the authority
to provide another body in its place. Notice that Ibrahim's faith was "credited as righteousness." His faith was seen in action when he was willing to offer his son, believing that God would still complete his promises to him. This is what God desires: For us to trust him...to show our faith through obedience. Read John 6:28-29 together. What act of obedience does God want from us? How can we show that we have this faith? 3. Tell the story of Musa (Moses). The angel of death was going to come through Egypt, bringing death. But Moses was sent to give this warning: If they slaughtered a perfect lamb and painted its blood on their doorposts, the angel of death would pass over their houses. When Jesus came, the prophet John announced immediately, “Behold! The Lamb of God who takes away the sins of the world!” Jesus is the lamb that God sent so that his wrath could pass over us. But like the Israelites, we have to
hear, believe, and choose humble, obedient faith. 4. Tell the story of Namaan (II Kings 5).Namaan had a skin disease that made him unclean. Elisha, the prophet of God, told Namaan to dip seven times in the Jordan River to be healed. Namaan was angry. But his servant girl asked this question: “If the prophet had told you to do some great thing, would you not have done it? How much more, then, when he tells you, ‘Wash and be cleansed’?”
The Bible says that sin is like a disease, and it makes us unclean. It makes us unable to be in the presence of God. Our good deeds are like bandages that we use to cover sin. But on the day of judgment, God isn’t asking to see our bandages – God is asking us to be clean. But God alone has the authority and the power to heal us. When we refuse his miracle of healing, and try to make up for our sin on our own, we are like Namaan: willing to do great things for God, but unwilling to submit to what God is asking. Like Namaan, we have to respond with faith and humility to accept the miracle of God. God said that this act of humility is of highest importance: He saves us "Not by works, so that no one can boast" (Ephesians 2:9).
You might ask your friend, "Muhammad spoke the words of the Qur'an approximately 4500 years after Azzam (Adam) was created. If God wanted us to outweigh our bad deeds with good deeds as a basis for salvation, why didn't he make this clear much earlier on?" Recognize that your friend might ask you the same about Jesus. That is: If the Gospel is true, why didn't he send al-Masih as our Adha right after sin came into the world? If your friend asks this, read Romans 3:24-26 and Romans 5:19-21 together. Look at this as a good opportunity to share Truth, not a question to be afraid of!
Ask your friend if he or she has heard that Islam allows a person to send someone else on Hajj in their place (by paying for their costs to go in person, they then receive the spiritual reward...including purification from all sin.) How is this a "just" form of substitution, in that it favors the rich, and appears to grant forgiveness without repentance? How is this acceptable in Islam, while substitution from faith, from a heart of repentance, is considered corrupt? Ask your friend to talk about what appears to be a double-standard. (Listen as they respond, and practice restraint...Don't rush to answer or appear combative.)