William Vallicella ranks the greatest philosophers, asking whether they were theists or atheists–and his list puts at least seven theists in the top ten. I think he’s asking the wrong question–an alternative question which would put things in a different perspective is
Were the greatest philosophers advocates of the mainstream religious views of the population they lived in, or did they express doubts about such views?
Atheism has always been a tiny minority view, especially if you only count those who called themselves atheists. Asking the question the way he does gives an advantage to theism simply in terms of number of candidates to select from. If he asked how many of the greatest philosophers were advocates of evangelical Christianity (of the sort that didn’t exist until the late 19th century), I’d expect the answer to be zero. Counting the greatest philosophers of history who were theists doesn’t tell you anything about whether theism is true–it’s a strategy much like that used by creationists who point out that many of the greatest scientists of history were creationists, populating it with names from before Darwin.