What is the best background to have before starting theological studies? Philosophy, without a doubt. After that I think it would be classics (because of the emphasis on Greek and Latin). Like many of my interests my philosophical background is spotty and unconventional. I took a couple of philosophy classes in college, and had to take several courses in seminary. Several good friends who studied more philosophy also helped provide something of an education in that area. Now I enjoy reading philosophy perhaps even more than theology. Overall, I find that it constantly helps me address certain questions that arise in various contexts - such as in classes at church.
My main interest for the past year or so has been Thomism. I have read a couple of books on St. Thomas, as well as some surveys. The surveys have been especially helpful because they situate St. Thomas in the larger context of Medieval philosophy, which helps you see the arguments on a broader level, and realize that the Angelic Doctor is not the last word on every subject. It is tempting, when a question in a class arises, to simply quote St. Thomas. But it is not accurate to suggest that his theory on a particular subject is the official teaching of the Church, which can happen if you are not careful! I learned my lesson on this when doing a class on angels at St. Mark's (very successful, by the way - that is a good topic to do a class on at church... people are fascinated by that subject). I said, following Thomas, that angels are not a species. Each angel is its own species because they are immaterial, and matter is the principle of individuation according to St. Thomas. But I later learned, in reading a book on Medieval philosophy that the Franciscan school (Bonaventure and Duns Scotus especially) did not hold to that idea. Angels were in different species (at least in Bonaventure... I am still studying Scotus). Not that anyone really cared... but it is a matter of integrity and accuracy!
Anyway, all of that is to say that one cannot nor should not pass off, either on purpose or by accident (no pun intended), certain philosophical theories on what are ultimately speculative matters as "church teaching". Still, being able to present a certain philosopher's explanation as a plausible one to consider on some subject (such as the where evil comes from) is helpful. Thus a priest should be conversant in philosophy.
