Adding more religious diversity to Turkey. After all, secularism arose in Europe in the first place to account for all of the conflicting religions/ denominations as an attempt to promote tolerance. With more religious minorities in Turkey, there will be greater call for secularism. Secularism will be seen as a tool of symbol of unity for various types of people in Turkey.
So, prevent all of those ethnic cleansing after and during World War 1.
Secularism became popularized by a country that was almost 100% Catholic. The Ottoman Empire, for example, managed to keep all its religious minorities pretty happy through a system of plularism (that is, religion was still an important part of government and society, but multiple religious structures exist in parallel). Most European countries followed something not too dissimilar. Pillarization in Belgium and the Netherlands, for example. And despite national diversity in Euoprean countries, basically every country but Germany, Russia, Switzerland, and the UK was more or less a single church (be it Catholic or Lutheran or whatever), and a smattering of Jews. And it's not like Russia was a paragon of religious tolerance. The UK was probably the most secular of those nations, and it still had an established church was fairly far-reaching powers (like
seats in the legislature).
In fact, even though he was a secularist, Ataturk was also very, very nationalist, and religion was tied up fairly heavily into that. The Greek-Turkish population transfer, for example, moved Christians to Greece and Muslims to Turkey, regardless of language and historical presence (for example, heavily Turkified Christians in Istanbul were expelled, as were completely 100% Greek families that had converted to Islam at some point over 500 years for whatever reason). Ataturk was OTL not super great with minorities (e.g., the Kurds), especially minorities that didn't want to assimilate.
Kemal won a war against Greece. If he is a secularist, shouldn't that impress the Turks? "If someone like him dares to defy Islam, secularism can't be all wrong"?
It might have helped if he wanted them to adopt another sect, but abandoning religion altogether? It's hard to make the argument, "Look what my lack of God has brought us! Clearly only by secularizing can we bring the great Seculos to bless us!"
