It's quite common for Muslim apologists to be inconsistent in their methodology. For instance, a Muslim will point to the Bible and say, "Ahhhh! You have textual variants! This is conclusive proof that your book has been corrupted!" Then, when Christians show that there are textual variants in the Qur'an, suddenly textual variants aren't important. Muslims will point to the Gospels and say, "Ha! Mark was written more than two decades after the life of Jesus! How can you trust such a late book?" Then these same Muslims will quote Sahih al-Bukhari as a trustworthy source on the life of Muhammad--despite the fact that it was written more than two centuries after Muhammad's life.
Muslims are especially inconsistent in their conspiracy theories. Conspiracy theories are meant to rule out evidence, so that the facts no longer count. For example, let's say that I'm a paranoid schizophrenic. "Everyone is conspiring against me," I cry. How could anyone ever prove me wrong? "David," you say, "I promise you that we're not conspiring against you. No one's plotting behind your back." This is meant to be evidence against my position, and yet I reply, "Ha! That's exactly what I would expect you to say! If you're plotting against me, of course you're going to deny it! Now I know that everyone is conspiring against me!"
Muslims apply conspiracy theories in numerous ways, e.g. "There was a conspiracy at the Council of Nicaea! They wrote the Gospels themselves!" But I want to focus on one particular conspiracy theory--the theory that Paul invented Christianity. In his debate with James White, Nadir Ahmed used this theory to rule out all the relevant evidence. Nadir claimed, without offering anything even remotely resembling a careful argument, that Paul had corrupted Jesus' message. Since the writings of the New Testament come after Paul, none of them can be used as evidence concerning Jesus' teachings. Moreover, since even the early church fathers were writing after Paul, their views don't count either.
Notice the difference between this conspiracy theory and a serious argument against Christianity. A serious argument would be based on the first century evidence. Nadir's conspiracy theory is meant to rule out everything that would normally count as evidence (and James pointed this out).
But does Nadir apply this theory consistently? That is, would Islam be able to deal with a similar attack? Let's find out.
I claim that Muhammad was a Christian. He believed in Jesus' death, resurrection, and deity. The vast majority of "Christians" in Arabia before the time of Muhammad were heretics. Hence, Muhammad came to restore true belief in Jesus Christ. He spent his entire life preaching the Gospel and turning people to faith in Jesus Christ. Many pagans were converted to Christianity under the powerful preaching of Muhammad and his disciples Abu Bakr and Umar. Muhammad gave his followers the Qur'an, which, in its original form, was simply an Arabic translation of the New Testament.
But there was an evil pagan named Uthman, who hated Christianity, didn't know Muhammad, and worshiped Allah, one of the many gods of Arabia. In order to destroy the work of Muhammad, Uthman pretended to be a faithful Christian. His deception was so convincing that he eventually rose to a position of leadership in the Christian community. Once he was in charge, Uthman asked all of the Christians in Arabia for their copies of the Qur'an (i.e. their copies of the Arabic New Testament). Uthman then rewrote the entire Qur'an, turning it into a book which denies the core teachings of Christianity. He took all copies of the true Qur'an and burned them, and he used his power to silence his enemies.
Thus, the Islam of today is not the religion that Muhammad preached. Muhammad preached submission to Jesus Christ. Uthman corrupted this message by claiming that Allah is the only true God and that Jesus was a mere prophet of Allah. He did this in order to degrade Jesus and to keep people from believing in Christianity, the religion of Muhammad.
Now for the fun part. How can Muslims refute my theory? They can't appeal to the Qur'an, since all copies of the Qur'an were written after Uthman corrupted them. But Muslims can't appeal to the Hadith, Sira literature, or commentaries either, since all of these were written after the time of Uthman. Muslims can't even say that Uthman was one of the companions, since I'm claiming that Uthman simply rewrote history to help his position. Isnad criticism is irrelevant, since later Isnad critics were under the influence of Uthman's false teachings.
I conclude that Muhammad preached Christianity and that Uthman was the true founder of what is now called "Islam."
My question to my Muslim friends is this: Do you really want to take the route of conspiracy theories?