Various places (for example, the books Reset: Iran, Turkey, and America's Future by Stephen Kinzer and From the Ruins of Empire: The Revolt Against the West and the Remaking of Asia by Pankaj Mishra) claim Ataturk said:
This is Islam, an absurd theology of an immoral Bedouin, a rotting corpse which poisons our lives.
and sometimes the book La mort d'un Empire (1954) by Jacques Benoist-Méchin is given as a source. Is there any truth in this?
I believe I have discovered an earlier source for this quote in the 1933 book Gray Wolf: The Life of Kemal Ataturk, by H. C. Armstrong.
I've skimmed through sections of the book and though I am not able to comment on the author too much going off some of the writing it seems to be pretty biased against Ataturk in particular for driving away the British empire. The book says in Chapter 49 (page 241 - 242):
To his friends he had always made it clear that he would root out religion from Turkey. When he talked of religion, he became eloquent and violent. Religion was for him the cold, clogging lava that held down below its crust the Earning soul of the nation. He would tear that crust aside and release the volcanic energy of the people. It was a poison that had rotted the body politic.
"For five hundred years these rules and theories of an Arab sheik," he said, "and the interpretations of generations of lazy, good-for-nothing priests have decided the civil and the criminal law of Turkey. They had decided the form of the constitution, the details of the lives of each Turk, his food, his hours of rising and sleeping, the shape of his clothes, the routine of the midwife who produced his children, what he learnt in his schools, his customs, his thoughts, even his most intimate habits. Islam, this theology of an immoral Arab, is a dead thing." Possibly it might have suited tribes of nomads in the desert. It was no good for a modern progressive State.
"God’s revelation!" There was no God. That was one of the chains by which the priests and bad rulers bound the people down.
“A ruler who needs religion to help him rule is a weakling. No weakling should rule."
And the priests! How he hated them. The lazy, unproductive priests who ate up the sustenance of the people. He would chase them out of their mosques and monasteries to work like men.
Religion ! He would tear religion from Turkey as one might tear the throttling ivy away to save a young tree.
Authentication of these is difficult as the author doesn't directly highlight the sources he referenced. The book does include a section titled "Works consulted and general references" (page 343 - 345) that has a list of sources. And the author himself mentions in the book ("Author's Note", page 10):
I HAVE been repeatedly asked whether the conversations quoted verbatim in Grey Wolf are actual or fictional.
Every quotation and conversation quoted verbatim in my Grey Wolf - with the exception of two which are of very minor importance and for which the evidence is less assured — has been supplied by Mustafa Kemal or obtained from documentary or verbal sources which have been severely tested and carefully weighed before their veracity and value have been accepted.
Some latitude must naturally be allowed in the wording as nearly ail are translations.
The source alluded to in the question, reliable or not, is Mustapha Kemal Ou La Mort D'Un Empire, which is the second volume of Le Loup et Le Léopard by Jacques Benoist-Méchin. No full English translation of this volume seems to exist.
According to a German blog post, the full quote attributed to Atatürk by Benoist-Méchin is as follows:
Depuis plus de 500 ans, les règles et les théories d’un vieux sheikh arabe, et les interprétations abusives de générations de prêtres crasseux et ignares ont fixé, en Turquie, tous les détails de la loi civile et criminelle. Elles ont réglé la forme de la constitution, les moindres faits et gestes de la vie de chaque citoyen, sa nourriture, ses heures de veille et de sommeil, la coupe de ses vêtements, ce qu’il apprend à l’école, ses coutumes, ses habitudes et jusqu’à ses pensées les plus intimes. L’islam, cette théologie absurde d’un bédouin immoral, est un cadavre putréfié qui empoisonne nos vies.
In Jihad in the West: Muslim Conquests from the 7th to the 21st Centuries (published in 1998) Paul Fregosi, gives an abridged English translation of this quote, citing page 323 of Le Loup et Le Léopard. The last sentence of his translation is "Islam, this absurd theology of an immoral Bedouin, is a rotting corpse which poisons our lives". I have not yet found any older English publication with a similar quote.
I'm in no position to evaluate Benoist-Méchin's work, but there you have it.
This doesn't address the question directly, but according to Andrew Mango's biography of Atatürk (I have the 2004 version), he said the following to Grace Ellison ("presumbably in 1926-27": Mango), which appeared in her 1928 book "Turkey Today" (1928).
I have no religion, and a times I wish all religions at the bottom of the sea. He is a weak ruler who needs religion to uphold his government; it is as if he would catch his people in a trap. My people are going to learn the principles of democracy, the dictates of truth and the teachings of science. Superstition must go. Let them worship as they will; each man can follow his own conscience, provided it does not interfere with sane reason or bid him act against the liberty of his fellow-men.
So, no reference to a "rotting corpse" (at least I can't find any in the Mango book), but quite scathing. This contrasts with Atatürk's actions earlier in his political career when Islam was specified as the official state religion of Turkey (early 1920s; hard to gauge from Mango, who also says Atatürk "was later to describe the reference as 'superfluous', explaining that [the designation of Islam as the state religion] had been tactically necessary at the time"). It ceased to be so in 1928.
