What to know If your friend has heard and understood the Gospel, they might struggle with Daoud's Psalm in light of the claim that Christ's sacrifice is necessary. If God could forgive Daoud, why he can't forgive us? Others may have seen Islamic memes that quote Psalm 51:16, asking Christians why they don't believe their book. (The verse says, "For You do not delight in sacrifice, or I would bring it; You take no pleasure in burnt offerings.") Daoud's assurance in his forgiveness, coupled with his direct statement that God doesn't delight in sacrifice, can raise legitimate questions to our Muslim friends. Why do Christians believe that sacrifice is necessary for forgiveness, given these verses? What to say
Look together to Romans 3:21-28, along with Acts 17:30. What does the Word of God speak into this question? Daoud received the forgiveness of God before the cross, but that "forebearance" was made possible because of the cross (Romans 3:25.)
If your friend continues to struggle over the Gospel-claim - that Jesus is a necessary sacrifice for our sins, and that without him, we are condemned - talk about the five attributes of God seen in the crucifixion: He is absolutely holy; he is absolutely just; he is completely merciful; he is completely loving; and he is all-powerful, with all authority.
Consider the implications: Could we say that God is just if he ignored sin; if he pretended not to see? Could we say he is holy if sin were insignificant to him? To believe that God is merciful and forgiving without the cross is to lessen his other attributes.
If your friend is willing, begin reading the Injil together to look at this in more depth.