Both Christianity and Islam claim to accurate report the truth about God. Whilst it is attractive to avoid conflict and suggest that ‘there’s truth in both’, when push comes to shove, this position is unsustainable unless you want to suggest that God doesn’t want things to be clear. Both faiths start from the claim of being revealed, which implies that God want to reveal certain truths.
The New Testament is very open to new revelation; prophecy is the only gift to make it into all three of the lists of spiritual gifts, and Paul tells us to seek the gift. The whole of Revelation is a prophecy, whilst Acts records prophets popping every so often. ‘Do not despise the words of prophets but test everything; hold fast to what is good’ (1 Thess 5 NRSV). Therefore it is unbiblical to dismiss the Qu’ran as a priori false; rather we need to look in more detail.
The obvious fail of Islam is its denial of the divine nature of Jesus. ‘Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son. 23 No one who denies the Son has the Father; everyone who confesses the Son has the Father also.’ (1 John 2) leaves little room for dispute at the obvious level.
However there are two other small but significant features that I’ve come across: the Hebrew Bible limits the number of lashes to 40 because. (Deut 25:3) ‘if more lashes than these are given, your neighbor will be degraded in your sight.‘ By contrast the Quran instructs: ‘The [Unmarried] woman or [unmarried] man found guilty of sexual intercourse — lash each one of them with a hundred lashes (24:2).
Similarly the OT instructs that a thief should pay twice the value of what is stolen, and if they are unable to do so, be sold into slavery. That sounds rough – but means that if there is no demand for his services, then he escapes without punishment; if he’s a poor man desperate for food for his family, his new owner becomes responsible for them… By contrast the Qu’ran’s instruction: ‘[As for] the thief, the male and the female, amputate their hands in recompense for what they earned [i.e. committed] as a deterrent [punishment] from Allaah. And Allaah is Exalted in Might and Wise. [5:38] So we have the disfigurement of God’s creation in response to theft, once again degrading the neighbour in your sight. Is that really the instruction of the creator God, or the musings of a successful business irritated at his losses to thieves?
I’m sure there are other, better, arguments out there. I offer these because to me they serve to challenge the whole basis of Islamic claim to worship the creator God who revealed himself to Abraham and the people of Israel.