The following thoughts came from a small discussion on facebook with a friend of mine.
First of all, in seeking to understand any other we need to understand their words, as they define them. We tell people that if they want to understand, properly, the Bible, that they have to understand the historical and cultural context of the authors, and understand what they meant, in the larger historical, cultural and textual context, by the words they used, rather than imposing our definitions on their terms. Furthermore, to truly understand the Bible, it is better to read it in the original language than to read a translation, because every translation is an interpretation The same thing is true in reading the ancient and medieval philosophers. Whenever we read a translation we read an interpretation, and, frequently we impose our definitions on their words.
Secondly, in order to truly be able to critique any body (whether it be an author or a friend we are talking with) we first need to be able to understand what they are trying to say. If we don't understand and interact with their thoughts, as they expressed them, and as they meant them to be understood, then we cannot critique them meaningfully.
Thirdly, though their may be many methods of understanding an author, I have found the following method to be the most fruitful.
(1) Read through everything the philosopher has said, noting their definitions, and the logical implications of what they are saying, based upon their definitions.
(2) Find the best commentators on the philosophers in question and read them, comparing your preliminary understanding of the philosopher, with that of the commentators. (This requires having the author that you are trying to understand beside you as you go through the commentator) The commentator will frequently give background information (cultural, historical, etc.). Don't read his commentary prior to reading the author in question. You may begin by reading the historical and cultural comments if it helps, but, I repeat, don't touch the commentary itself until after you have read the works of the author that you are seeking to understand!!!
(3) Read other books on the philosopher that are "systematic theology" type books, in other words, books that don't comment on the the author's writings, but seek to explain systematically the author's system.
(4) Read books by people who disagree with the philosopher and see if their critique's hold water.
If you change the order of this 4 step method, you will seriously limit your capacity to truly understand the author in question, you will taint your ability to objectively analyze and critique the author in question, and you will end up reading the author in question through the lense of whichever commentator, sysematiser or critique that you read first. A great book to read on this subject is Mortimer J. Adler's How to Read a Book.
So, for example, if we apply these four steps to Aristotle, we would advance as follows:
(1) Get ahold of the Loeb Classical Library versions of Aristotle's works. They are printed with the greek text on one page, and a good translation on the opposite page. All of Aristotle's works are printed in this series. For the Metaphysics which the primary source for Aristotle's Metaphysical and Epistemological thoughts you should get Werner Jaeger's Greek edition of the Metaphysics published by Oxford, and a couple translations (W.D. Ross is one of the classic translations.)
(2) By far the best commentator on Aristotle's works (by almost universal agreement) is Aquinas. He commented almost all of Aristotle's works, and you can get a really good translation of these commentaries from Dumb Ox books. Otherwise, you should get the Marietti latin versions. Aside from Aquinas, Heidegger wrote some interesting commentaries on Aristotle, as did a number of medieval authors.
(3) For systematizers, you need to go by subject, so, for Aristotle's Metaphysical (and the foundation for his epistemology) theory, my personal preference is Joseph Owens "The Doctrine of Being in the Aristotelian Metaphysics", and Joseph Owens, "Aristotle's Gradations of Being in Metaphysics E-Z". Another classic is C. D. C. Reeve, "Substantial Knowledge: Aristotle's Metaphysics". Two interesting books on Aristotelian epistemology (though not the absolutely best) are the book edited by John Wild, "The Return to Reason", and John Wild, "Introduction to Realistic Philosophy". On Aristotelian Ethics see Henry B. Veatch "Rational Man: A Modern Interpretation of Aristotelian Ethics", and the book of essays "Essays on Aristotle's Ethics", edited by Amélie Oksenberg Rorty. It is always helpful to have the Cambridge Companion to Aristotle. Anything by Jonathan Barnes on Aristotle will be worth reading.
For (4) any modern platonist or Augustinian will do. I have found that most Christian apologists who criticize Aristotle are very superficial to the point of being factually false. I was very disturbed by some comments concerning Aristotle by one of my favorite Christian Apologists, John Lennox, in his book Gunning for God, as they were factually inaccurate.
If we apply this procedure to the Bible it might look something like this:
(1) Get ahold of a good Greek version of the New Testament (for example: The Greek New Testament, 4th revised edition, published by the United Bible Societies, the Nestle-Aland Novum Testamentum Graece, and The Greek New Testament According to the Majority Text by Zane C. Hodges/Arthur L. Farstad). A good interlinear would also be helpful. Get ahold of a good Hebrew/Aramaic Old Testament, and a good interlinear. (Some good greek language tools such as Perschbacher's New Analytical Greek Lexicon, and the BDAG Lexicon are the best you can get for english readers.)
(2) Get ahold of the commentators. This, of course, is alot of books, as each individual book in the Bible has been commented on by 100's of commentator from the time they were written to now. So, you'll have to do some research to find out who are the best on each book. For the book of Romans you can't go wrong if you get ahold of, at least, Douglas Moo (NICNT series and NIV series), Sanday & Headlam, Cranfield, Frédéric Godet, Luther, Aquinas and Chrysostom.
(3) For the Bible you want to look for good systematizers, again, there 1000s of theology books. But, some that are absolutely necessary would include, Aquinas's Summa Theologiae, Hodge's Systematic Theology, Calvin's Institutes, Barth's Dogmatics, Geisler's Systematics, Augustine's Confessions and City of God, and a multitude of other works that touch on only one or two subjects, but, the list so long that it is hard to do it justice in a short post. (I have another page where I am developing a list of recommended books according to subject. I am constantly updating it, and would refer the reader to this page.)
(4) The list of critical books is also so long that it is not worth listing it. Any book by an atheist or agnostic or muslim that purports to destroy some christian doctrine would count. Of course there are certain authors that aren't worth reading, such as Richard Dawkins and many of the new atheists (as their critiques are anything but serious), and other authors that are worth reading, such as Mackie and the young Flew.