Depends on what you mean by Turkey. If you are asking the religion of the people living in modern Turkey, then the answer would be Christianity. In fact, close to orthodox Christianity, because Anatolia was under Byzantine rule before Islamic conquests.

If your questions regards pre-Islamic Turkic/Turkish people, then I would say mostly Tengriism, but there were also Buddhist Turkic people as well as Manicheaens, and people believing in Judaism (Khazar Turks).

It depends on the exact period you're looking for since Asia Minor has changed hands many times. In general though, it's religious history was shaped by it's integration into the Roman, and later Byzantine empires.

As such, Orthodox Christianity would have been the major organized religion.