[Gratia]

Could anyone explain what this has to do with the Overton window?

Seems like a fancy concept thrown in for no particular purpose, in the midst of a gravely serious topic.

[hinckley buzzard]

Re: the Overton Window, there is less there than meets the eye. It was beloved by the pseudo-wonk Glenn Beck back in the day, when he wanted desperately to appear to have some arcane insight into the world. All it means is the set of ideas and memes that are acceptable in the general discourse to a given population at a given time.

[Albion Wilde]

The generic term "Overton window" refers to "the range of policies politically acceptable to the mainstream population at a given time. It is also known as the window of discourse. The term is named after American policy analyst Joseph Overton." (Wikipedia)

So, with reference to the OP, the range of allowable speech about Christian concepts is narrowing in Europe, as it is here. I think it may be worse over there, from what I've read.