And if ye are in doubt as to what We have revealed from time to time to Our servant, then produce a Sura like thereunto; and call your witnesses or helpers (If there are any) besides God, if your (doubts) are true.
-- Qur'an 2:23 (trans. A. Yusuf Ali)
Although as a Christian I certainly can't claim to be impartial in matters of religion, I have always been a little put off by the challenge in Qur'an 2:23.
A minor objection is that aesthetics can never be a measure of truth. Although truth can certainly be beautiful, so can lies. On the other hand, truth can sometimes be unattractive - like an uncut diamond, or a boring sermon.
Still, I'm not sure that's a major obstacle. It could plausibly be argued that Allah was not making beauty the measure of truth, so much as he was making the Qur'an's beauty a sign to doubters. It's a subtle distinction, but I think it's valid - not unlike Moses besting the sorcerers of Egypt with his miracles. Miracles never became the standard of truth in a logical sense (most of us have probably heard of "lying wonders"), but Moses's miracles were a sign that Allah was with him.
So the inappropriateness of aesthetics as an arbiter of truth is not my major objection. The real problem is much graver. In particular, I believe that the main problem is that this is not a legitimate challenge at all, because the deck is heavily stacked in favor of the Qur'an.
There is a subtle but very real circularity for believing Muslims. The circularity goes like this: If you're an orthodox Muslim, you believe that the entire Qur'an is the very word of God, and is therefore inerrant. As an implication of this, he or she also believes that there is no other writing as eloquent as the Qur'an. Therefore, any attempt to "produce a Sura like thereunto" must fail - thus verifying that the Qur'an is the very word of God and inerrant. In fact, the verse following the challenge affirms the impossibility of producing such a Sura, and threatens disbelievers with the Fire whose fuel is men and stones:
But if ye cannot - and of a surety ye cannot - then fear the Fire whose fuel is men and stones,- which is prepared for those who reject Faith.
-- Qur'an 2.24 (trans. A. Yusuf Ali)
It is no surprise, then, that Muslims have found no sura equal to the Qur'an. The surprise would be if they had, since even admitting the possibility of a better Sura contradicts 2:24, which says "of a surety ye cannot," hence being a sign of disbelief.
When I see a challenge like this it does not convince me of the Qur'an's truth. It does rather the opposite, making me suspect that the message is false. (By the way, I mean no offense to any Muslims who may read this. I just couldn't think of any inoffensive way to say this.)
It seems to me that the real test of truth is how well it stands up when stripped of eloquence:
When I came to you, brothers, I did not come with eloquence or superior wisdom as I proclaimed to you the testimony about God. For I resolved to know nothing while I was with you except Jesus Christ and him crucified. I came to you in weakness and fear, and with much trembling. My message and my preaching were not with wise and persuasive words, but with a demonstration of the Spirit's power, so that your faith might not rest on men's wisdom, but on God's power.
-- 1 Corinthians 2:1-5
Eloquence is a good thing, a powerful servant of truth. But it is also fickle, serving falsehood as eagerly as truth.
So, how well do the messages of Islam and Christianity stand up on their own merits? Now that's a question worth debating.
Overview on the question: Is the Qur'an miraculous?
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