|American Robert Spencer's 2012 Did Muhammad Exist?, in which he recounts how he unlearned his previously held 2007 belief that Muhammad was a real historical figure. |
In 1239, Frederick II published a treatise that denied the divinity of Jesus, Moses, and Muhammad, declaring each of them imposters.
In 1930, Soviet Marxist theoretician Liutsian I. Klimovich, in his lecture “Did Muhammad Exist?”, argued the time gap between Muhammad’s alleged lifetime and the first written sources was so huge that we cannot suppose that any of the information given in these sources is authentic; that nothing is known for sure about the historical Muhammad, and that it is even likely that he never existed. Quite consequently, Klimovich assumed that the Koran was not Muhammad’s work but the product of a whole group of authors. Muhammad was created by later historians as a myth, designed to explain the emergence of the Islamic community out of the Hanif movement. The prophet was an invention to cover up early Islam’s character as a social protest movement. 
In 2003, Walter Williams, in his The Historical Origin of Islam, asserted that Muhammad is the brainchild of Arabian scholar Ibn Arabi (1165-1240) (Ѻ) who was the main individual involved in the morphing and mythologizing (Ѻ) of the making of the prophet Muhammad; he also shows that the the Koran is a mixture of Jewish literature, put together by Jewish scholars to include the Torah and the New Testament Gospels. 
In 2007, Robert Spencer, founder of Jihad Watch (2003), author of The Politically Incorrect Guide to Islam (2005) and The Truth About Muhammad: Founder of the World’s Most Intolerant Religion (2006), was at a conference with Ibn Warraq, author of Why I Am Not a Muslim (1995), during the course of which they whet for walk together, discussing Islam, during which time Warraq made the following comment:
“I don’t think Muhammad ever existed at all.”— Ibn Warraq (2007), comment to Robert Spencer during walk 
Spencer, who at the time believed that Muhammad was a "real person", who actually existed as an historical figure, i.e. he believed Muhammad was a "false prophet", someone who sold his own opinions off to people as divine revelations, and Warraq engaged into debated on issue. Warraq's reasoning and answers paused Spencer into reflection. In the next five years, Spencer came to reevaluate his position:
“Ibn Warraq is responsible for the fact that I no longer believe in Muhammad. I no longer believe Muhammad existed as an historical figure.”— Robert Spencer (2013), “Interview with Ibn Warraq” 
In 2008, German jurist and former Islamic theologian Sven Kalisch, whose doctoral thesis (1997) was on “common sense and flexibility in Islamic legal methodology” (see also: Goethe's jurisprudence issues and student reactions), completed at the Faculty of Law and Economics at the Technical University of Darmstadt—a Protestant-to-Muslim convert (age 15) turned Muslim-to-nonbeliever convert (age 44)—made controversial headlines by stating, in an article entitled “Islamic Theology Without the Historic Muhammad: Comments on the Challenges of the Historical-Critical Method for Islamic Thinking”,
“Muhammad probably never existed.”— Sven Kalish (2008) (Ѻ)
Kalisch's researched opinion is that Muhammad never existed; at the time, he was said to be in the middle of completing a book on this subject. 
In 2011, Hans Jansen, in his “The Historicity of Muhammad, Aisha and Who Knows Who Else”, argued the position that Muhammad is a fictional character. 
“It may sound crazy but it is not as crazy as it sounds: a number of scholars consequently suspect that Muhammad is not a historical figure, but a literary character that was created by ancient Arab storytellers, perhaps early in the eighth century of our era.”— Hans Jansen (2011), “The Historicity of Muhammad” 
In 2012, Robert Spencer, in his Did Muhammad Exist?: An Inquiry into Islam's Obscure Origins, asserted, per reason that there is no mention of a person named "Muhammad" in Arab literature until 692, a full sixty years (Ѻ) AFTER the purported death of Muhammad in 632, that what is Islamic revelation was an afterthought, a narrative created after the Arab conquests of the Near East, to give the new ruling elite an ideological pretext for power. 
In 2013, Karl-Heinz Ohlig (1938-) (Ѻ), in his “From Muhammad Jesus to Prophet of the Arabs”, argued that the term “Muhammad” was not an actual person, but rather an epitaph, meaning “praised one”, “promised one”, or “god’s servant”, and agues that the four mentions of the name Muhammad in the Quran, do not justify the existence of an actual extant person, but rather one or more series of Muslim preachers who were selling an new reformed “Muhammad Jesus” figure as role model for a new Jewish-Christian upgraded religion, or something to this effect, in sort. 
The general issue, to summarize this general Muhammad did not exist view, is that all of the main figures, characters, and prophets of Judaism, Christianity, and Islam, each of which progressively building on the other, are human character re-writes, aka "god reduction" morphs, of what were primarily original Egyptian mythological gods and stories, as the works of the 100+ religio-mythology scholars have shown, e.g. Noah is the god Nu (Noah never existed), Abraham is the god Ra (Abraham never existed), Sarah is the star Sirius, Virgin Mary is the goddess Isis, Jesus is the god Osiris (see: Jesus never existed), Joshua is the god Shu (see: Joshua 10:13), John the Baptist is the god Anubis, Joseph is the god Geb, and so on (see: Egyptian-to-Christian transliteration); hence, if the Islamic figure Muhammad claimed to have "spoken" to all of these so-called prophets, e.g. during his famous "night journey", then he too is a mythological figure, sold to people, over time, as a real person, as a way to found Islam on the historical religions, aka ancient mythologies.
● Noah never existed
● Abraham never existed
● Sarah never existed
● Buddha never existed
● Moses never existed
● Jesus never existed
1. (a) Existence of Muhammad (section) – Wikipedia.
(b) Klimovich, Liutsian. (1931). “Did Mohammed exist? The debate in the Communist Academy in anti-religious section of the Institute 12/1930 CI philosophy. on the report of LI Klimovich” (“Sushchestvoval li Mokhammed? Diskussiia v Kommunisticheskoi akademii v antireligioznoi sektsii instituta filosofii 12/XI 1930g. po dokladu L.I. Klimovicha”), in: Voinstvuiushchii ateizm, No. 2-3, (1931), 189-218.
2. Williams, Walter. (2003). The Historical Origin of Islam (abs). Maathian Press.
3. (a) Spencer, Robert. (2013). “Ibn Warraq Exposes Islam on ABN” (Ѻ), Betsy Ross, Jan 31.
(b) Robert Spencer (author) – Wikipedia.
4. (a) Kalisch, Sven. (2008). “Islamic Theology Without the Historic Muhammad: Comments on the Challenges of the Historical-Critical Method for Islamic Thinking”, (in German), Publication.
(b) Anon. (2008). “Excerpt: Muslim Academic Questions Muhammad’s Existence”, The Wall Street Journal, Nov. 15.
(c) Higgins, Andrew. (2008). “Professor Hired for Outreach to Muslims Delivers a Jolt” (Ѻ), The Wall Street Journal, Nov. 15.
(e) Historicity of Muhammad – Wikipedia.
(f) Thims, Libb. (2011). Purpose? (in a Godless universe). (94-pg manuscript) (unfinished); Online as 105-page unfinished manuscript (14 Apr 2013). IoHT publications.
(g) Sven Kalisch – WikiIslam.net.
(h) Sven Kalisch (German → English) – Wikipedia.
5. Jansen, Hans. (2011). “The Historicity of Muhammad, Aisha and Who Knows Who Else” (Ѻ), Trykkefrihed.dk, May 16.
6. (a) Spencer, Robert. (2007). The Truth About Muhammad (Ѻ). Publisher.
(b) Spencer, Robert. (2012). Did Muhammad Exist?: An Inquiry into Islam's Obscure Origins (foreword: Ibn Warraq). Open Road Media.
(c) Zmirack, John. (2012). “Amazon Review” (Ѻ), Apr 9.
7. Ohlig, Karl-Heinz. (2013). “From Muhammad Jesus to Prophet of the Arabs: the Personification of the Christological Epithet” (pdf), in: Early Islam: a Critical Reconstruction Based on Contemporary Sources (251-307). Prometheus.
● Spencer, Robert. (2015). “Did Muhammad Really Exist? No!” (Ѻ), Keep it Real Israel, Nov 19.