First, start with this background piece from City Journal in 2012. Note, especially, this:
Gülen’s detractors, however, inevitably point to a speech of his that surfaced in a video in 1999:
You must move in the arteries of the system without anyone noticing your existence until you reach all the power centers. . . . Until the conditions are ripe, they [the followers] must continue like this. If they do something prematurely, the world will crush our heads, and Muslims will suffer everywhere, like in the tragedies in Algeria, like in 1982 [in] Syria, . . . like in the yearly disasters and tragedies in Egypt. . . . The time is not yet right. You must wait for the time when you are complete and conditions are ripe, until we can shoulder the entire world and carry it. . . . You must wait until such time as you have gotten all the state power, until you have brought to your side all the power of the constitutional institutions in Turkey . . . . Now, I have expressed my feelings and thoughts to you all—in confidence . . . trusting your loyalty and secrecy. I know that when you leave here, [just] as you discard your empty juice boxes, you must discard the thoughts and the feelings that I expressed here.Then read this recent article entitled "Turkey's Two Thugs".
And then, for some more detail into the recent arrests, read this article from Reuben Silverman.
Related Posts: The False Prophet; More on Fethullah Gulen