Many midieval writers on Muhammad and Islam came straight out and said that Muhammad was an anti-Christ. One of the earliest non-Islamic accounts of Islam,
Doctrina Jacobi (c.640), tells of a conversion between a Jewish forced convert to Christianity and an elder of the Palestinian community that is written as follows:
When the candidatus was killed by the Saracens, I was at Caesarea and I set off by boat to Sykamina. People were saying “the candidatus has been killed,” and we Jews were overjoyed. And they were saying that the prophet had appeared, coming with the Saracens, and that he was proclaiming the advent of the anointed one, the Christ who was to come. I, having arrived at Sykamina, stopped by a certain old man well-versed in scriptures, and I said to him: “What can you tell me about the prophet who has appeared with the Saracens?” He replied, groaning deeply:
"He is false, for the prophets do not come armed with a sword. Truly they are works of anarchy being committed today and I fear that the first Christ to come, whom the Christians worship, was the one sent by God and we instead are preparing to receive the Antichrist. (
source; emphasis mine)
From the beginning, it appears that some Christians recognized not only the falsity of Muhammad’s prophethood, but his preaching not of the true God but of an anti-Christ figure. (Full disclosure: It is not clear from the extract presented above that the old man mentioned above was in fact a Christian, but as the account was written by a Christian who himself used the term “Antichrist”, I would say it counts.)
Alvaro of Cordoba followed suit in the 800s, as did others. Even others who held that it was, strictly speaking, a Christian heresy (which I assume to be the majority position held by non-Muslims during Islam’s period of expansion and rule over much of Europe, when a great many such polemics appeared in the West) ultimately concluded that Muhammad was at least a precursor to the anti-Christ, as Peter the Venerable (10th century) did. I guess the question is now is whether or not a/the anti-Christ ultimately worships the one true God…and that question makes my head hurt more than anything else we’ve discussed.
We might then also ask what exactly is meant by “heresy”. If Islam is in fact a Christian heresy (I don’t have a definite opinion this myself), does it follow that a heretic cannot worship the one true God? From the Catholic Encyclopedia article on the noun in question (available
here):
I’ve bolded portions that I think most apply to Muslims. They refuse to believe in Christ, of course, and even as they say that this is not in fact true (since they have their “Isa” in the Qur’an), they still restrict belief in His doctrine to that which harmonizes with their own book and its doctrines, the source of which is ultimately something other than the one true God.