Since I am constantly urging theologians and biblical interpreters to read philosophy, I frequently get asked what to read in philosophy by those who feel a need to know more about philosophy. There are no shortcuts. If you started an M.Div. without the equivalent of a minor in philosophy, you were at a disadvantage from Day One. Up until a few decades ago philosophy and Greek were required by good seminaries for entrance. Now only a minority of seminary students have studied philosophy very much if at all.
Maybe that describes you. So, what do you do now?
If you have never taken a philosophy course, you need four things: 1) a basic understanding of the history of philosophy and how it has influenced theology with an emphasis on ancient Greek philosophy; 2) an introduction to metaphysics, 3) a survey of ethical theory and 4) a course in Aristotelian logic. This is the basic minimum; it would be a good minor in philosophy in a B.A. program.
My advice, as a practical matter, is to ignore the modern stuff, that is everything from Descartes on, initially. You can come back to it later. An introductory survey of the whole 2500 years is a fine starting point. But what you really need to understand the development of Christian theology is a basic grasp of how four major figures have influenced Western thought: Plato, Aristotle, Augustine and Thomas Aquinas. The writings of Plato and Augustine are more accessible than those of Aristotle and Thomas. Fortunately, however, the latter have great expositors who can lead you through their writings.
Just signing up for whatever your local university or college offers in philosophy is a bad idea. Many college philosophy departments teach sophistry and not actual philosophy. (I regard postmodernism as basically a revival of ancient sophistry.) Stay away from it. Choose professors who focus on ancient and medieval figures as their own main areas of research if you have nothing better to go on. That might mean they actually care about philosophy. It is much easier to understand modern philosophy if you know ancient philosophy well. But just knowing modern philosophy will not allow you to understand ancient philosophy without a lot of work and may convince you that you don’t need to know the old stuff.
The internet is a rich resource, but it is like a flea market. You have to paw through the junk to find something valuable. I suggest the following books initially. But sooner or later you need to find a teacher.
Edward Feser has a blog and many books that are clearly written on the basis of his vast knowledge of philosophy. He is a Thomist and a former atheist. He works in analytic philosophy, but does not let it deter him from seeing the truth in classical metaphysics. He excels in presenting classical metaphysics in modern terminology paying attention to modern objections. He is very sharp, but he does not suffer fools gladly. The Last Superstition: A Refutation of the New Atheismsurveys the history of philosophy in a couple of hundred pages and it is a great place to start.
His book, Five Proofs of the Existence of God, demonstrates that God's existence can be proven by reason. It covers the proofs of Aristotle, Plotinus, Augustine, Thomas, and Leibniz. His little introduction to Aquinas (creatively entitled: Aquinas: A Beginner’s Guide) is a short introduction to Thomas Aquinas that is as easy to understand as it gets. Read it after The Last Superstition and before Five Proofs.
Recent writers who are more indebted to Plato and/or Augustine include C. S. Lewis, Lloyd Gerson and Peter Kreeft. Gerson's trilogy on Platonism, defines the central tradition in philosophy. It consists of Aristotle and Other Platonists (2005), From Plato to Platonism (2013) and Platonism and Naturalism: The Possibility of Philosophy (2020). This trilogy explains how Platonism was the central tradition in ancient philosophy and how modern philosophy differs from it. The great little book by C. S. Lewis, The Abolition of Man, is pure gold. His analysis of the effects of nominalism takes on greater significance when read in the light of Gerson’s work. Also, see Peter Kreeft's book, The Platonic Tradition.
Much of the best philosophy has always been done outside university departments of philosophy. C. S. Lewis, for example, never taught philosophy. Neither did G. K. Chesterton. Some of the great novelists (like Dostoevsky), political thinkers (like Leo Strauss) and poets (like Wordsworth) are serious philosophers. Philosophy is "love of wisdom" and as Pierre Hadot has stressed, it is not merely an academic discipline, but a way of life. Actually, we are all philosophers; few of us, however, are good ones.
In terms of the history of philosophy, I would recommend W. K. C. Guthrie's A History of Greek Philosophy. In 6 vols. he covers the pre-Socratics to Aristotle. If you think this is overkill, you know nothing of philosophy. A. N. Whitehead said all philosophy is a footnote to Plato and that is only a slight exaggeration. F. C. Copleston’s multi-volume history is a basic reference work. I look forward to reading Peter Kreeft’s four-volume history of philosophy.
All truth is God's truth.
(This post is an expansion of a Twitter thread from the other day.)