[dr_lew]

If you would say he was not devout, I would agree. But as he so freely and naturally referred to God, as in “Raffiniert ist der Herr Gott ...” it’s hard to tag him as an atheist.

[FredZarguna]

There are direct and unequivocal comments which make clear that he did not believe in a personal God. I think some of his statements imply he did not believe that God was even directly "knowable," even if His creation was.

I'm not sure there is an excluded middle between "not a Theist" and "atheist" in actual usage. Apparently you do, and that's a sensible position. I did not mean to imply he was an atheist, but in any sense in which believers in the God of Abraham would see God, he was not a believer.