Grace Ellison was a British journalist who interviewed Mustafa Kemal Atatürk in person. She described one interview in her 1928 book Turkey To-day. This interview would be quoted by Andrew Mango in his 2011 biography titled 'Atatürk' to illustrate Atatürk's secular and rationalist credentials.
Of particular relevance here are her questions about the modernising reforms that he was proposing and his responses to those questions (pp23-24).
"... In two years’ time every woman must have her face uncovered and work side by side with men; and the men will wear hats. The day when clothes were the symbol of a religion has passed. The fez which symbolized a faith despised by Western civilization must go, and all the fanaticism that goes with it!"
Ellison raised the question of how the hodjas [Muslim schoolmasters] might react to his reforms. Atatürk replied:
"The hodjas! Indeed you are right! We have been a priest-ridden nation too long. Our reverend friends must learn to behave themselves. If they refuse, — well, they can always join the Sultan."
Ellison went on to quote Atatürk as saying:
"You speak of religion," said he, when I had expressed my doubts as best I could. "I have no religion; and at times I wish all religions at the bottom of the sea."
This would certainly suggest that he self-identified as an atheist or, at the very least as an agnostic.
However, notwithstanding that last remark, it is clear that Atatürk had no wish to impose his own views on others. Ellison continued:
"He is a weak ruler," said the Ghazi, "who needs religion to uphold his government; it is as if he would catch the 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 ; every 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."