It used to be, in Western society, that a philosopher was also a theologian, and a mathematician, and a literary critic. A philosopher was just about anyone who wrote for a living who wasn't also a poet, and sometimes someone was both a philosopher and poet, like Dante, for example. It's not well-known that Galileo wrote commentaries on Dante, but in his time it didn't seem strange -- he was a learned man, everyone agreed on that. Why shouldn't he write commentaries on Dante? The fact that the philosopher Roger Bacon in the thirteenth century, and Descartes, Spinoza and Leibniz in the seventeenth and earlier eighteenth centuries, were also leading mathematicians of their day, did not seem at all remarkable to their contemporaries -- they were philosophers. Who else but a philosopher should lead the way in math? The fact is that the term philosopher meant something very different back then, it referred to a learned man, and a learned man was expected to study all fields of learning. The division of labor which makes it seem strange that an astronomer is also a literary critic, or that leads some people to claim that a biologist like Richard Dawkins is not competent to write on theological matters, because he is a biologist, is a recent intellectual habit in our society, not more than a few centuries old.
Nowadays, a philosopher is -- what? Philosophy is a rather ill-defined term today. I think it's defined negatively, by the things which it is not, by the disciplines which have broken away from it. Philosophy is no longer astronomy or chemistry or mathematics, although the combination of philosophy and mathematics lasted somewhat longer than the combination of philosophy and some other fields. (A philosopher can of course still be an astronomer or a chemist. The difference is that now it would seem odd.)
Theology has not yet completely broken away from philosophy, or should I say, philosophy has not yet completely freed itself from theology. This is good for the reputation of theology and bad for that of philosophy.
One of the chief tasks of theology, a task which has grown steadily in importance over the past couple of centuries as atheism has begun to spread like wildfire, is to KEEP THINGS MURKY.
CLARITY is an archenemy of religion. And so when you make some clear points in a public forum about religion, and it's clear as well that you have the Abrahamic religions in mind, and above all contemporary Christianity in the US, there's a fairly good chance that some theologically-minded individual will come along and accuse you of having said something which does not apply at all to the Upanishads. And it's not unheard-of that this individual would be a professor of philosophy. Faculty in both philosophy and theology will bore and infuriate you with long speeches closely resembling sermons, and they'll make things even worse by enthusiastically quoting people like Nietzsche and Freud. Nietzsche hated, hated, hated theology and was crystal-clear about that, Freud took for granted that his stuff was not to be mixed up with that stuff those jokers down the hall in the theology department were instigating. Both Nietzsche and Freud underestimated how low theologians would stoop. They're like that repulsive booger which has attatched itself to to the end of your finger, and you shout in horror and shake and shake your arm and hand but it stays stuck there.
Life can be confusing under the best of conditions, and when it comes to philosophy there is often the difficult attempt to re-define certain things most of us take for granted, there are often long or rare words and texts in many different languages. But don't let the long words and various languages of theology fool you, philosophy does not have to be lumped in with theology. Schopenhauer, Marx, Nietzsche, Heidegger, Adorno, Sarte, Derrida & co are atheists, they aren't having any of that stuff -- although some of them do often cite authors of the time of the Christian hegemony, also known as the Late Ancient, Medieval and Renaissance eras, and show how substance and sense con be separated from the obligatory religious goobledeegook of those times.
With recent theology, the division of labor has proceeded to the point, I fear, that the goobledeegook has become their whole profession. Kierkegaard may mark the end of the era where philosophy and theology were still combined. (Karl Barth, Karl Barth! they're shouting. No. I really don't think so.)