St. Augustine: Confessions: This is a saint of the catholic church who lived in the 5th century A.D. He had a sharp mind and a great style, and the Confessions is much like an autobiography. He wrote a lot more that I mostly did not read.

St. Thomas Aquinas: Summa contra Gentiles, Summa Theologicae: This is a saint of the catholic church, and its most important theologian and philosopher. He had a great mind, and both summas repay reading (if perhaps in part), simply because he was so clever and made many subtle distinctions. The Summa contra Gentiles - Theses against the Heathens - is earlier and shorter than the Summa Theologicae. 

Duns Scotus: Philosophical Writings: John Duns, the Scot, was known as the subtle doctor, and indeed he was very bright. The work I list gives several of his shorter writings concerning God with a good English translation of the Latin text on the facing page.

Ockham: Summulae Logicales: William of Ockham was another very bright mind. The work listed is his extensive and interesting work on logic, partially translated by Loux and others into English, with long and useful introductions.

Of the four very bright catholic theologians mentioned in this file, Ockham seems the least doctrinal catholic, and also the one who is most interesting outside theology, because of his ideas about logic and about universals.

Marsilius of Padua: Defensor Pacis: Marsilius was a friend of Ockham, and the short work mentioned, published 1324, proposes to separate State and Church for very sound and well argued reasons.

The following philosophers are Renaissance philosophers, rather than that they properly belong to the Middle Ages, and they also are far less theological, doctrinal or believing than those mentioned so far.

Also, the ones I mention - Machiavelli, Montaigne and Bacon - all wrote a great prose style, unlike the abovementioned Scholastics, apart from St. Augustine.

Machiavelli: The Prince, The Discourses: Both books are about politics, which Machiavelli attempted to set on a new, empirical, basis. There is an edition of The Prince with my comments on this site, but it should be mentioned that The Discourses is a longer and clearer work that probably gives more of Machiavelli's true opinions than The Prince. If you want to have realistic ideas about politics and politicians you should read Machiavelli.

Montaigne: The Essays: These are three volumes and both great writing and great thinking. Montaigne was probably a skeptic (the "probably" is due to the fact that some claim he was a catholic who defended the catholic teaching through skepticism). The Essays are not systematical philosophy, but touch many philosophical problems, and also have a lot about human psychology and what it is to be a human being. There is a very fine English translation of 1608 by John Florio in 'Everyman's Library', that Shakespeare also is supposed to have read.

Bacon: Essays: This is a short book of very well-written essays, possibly in imitation of Montaigne.