The Fount of Knowledge is like an ancient theology encyclopedia. Written by St. John of Damascus some time shortly after 743 at the request of a friend who had recently become bishop, it summarizes Aristotelian philosophic terminology, lists a little over a hundred heresies (including Judaism and a long section on Islam), and spends the last ⅔ of the book on theology, the part known as On the Orthodox Faith (specifically, An Exact Exposition of the Orthodox Faith; thanks for assuring us that it is exact, John, nobody writes A Vague and, In Places Incorrect, Summary of Theology on purpose). The theological discussion synthesizes eastern Patristic thought, frequently citing St. Gregory of Nazianzus and St. Bazil, but also some other eastern writers. According to the introduction, John is the last eastern theologian. My edition, translated by Frederic Chase in 1958, offers extensive footnotes giving precise locations of what John is citing. This edition also has a table of contents which lists the topics, which is helpful for using this book as a theological reference.

I recommend reading the philosophical chapters before reading the theology, unless you are already familiar with Aristotelian terminology, since the theological arguments require you to know the differences between substance and nature and other similarly nuanced terminology. The modern English meanings of the words are very different. Substance, for instance, is roughly equivalent to Plato’s Form, which is decidedly insubstantial in modern terminology.

The summarizing of the heresies is more interesting that one might think. Most of them are obsolete heresies no one except scholars will even have heard of, but the list also includes some heresies that might be familiar, like the Nicholaitans (mentioned in Revelation). John summarizes the various schools of Judaism, and also offers a number of refutations of Islam—in whose domain his Syrian monastery was located. So this summary gives some historical insight.

The theological section spends over half the space arguing about God’s nature (“nature” in the modern sense), basically a logical argument for Nicene trinitarianism. To my mind this is both overly esoteric, since it relies on the ancient understanding of souls, but also unnecessary for anyone who accepts the Nicene Creed. John is thorough, though, so he supplies plenty of argument. There is also a discussion of ancient cosmology, such as the seven spheres and where each is located (the moon closest), and why God put the waters outside the firmament which contains the spheres (to keep the firmament cool). John does discuss other theological topics, such as the Fall and the Resurrection after the Second Coming, but these are discussed briefly.

Unlike John, I will offer an incomplete and quick possibly erroneous listing of interesting theological points. I am not going to include most of the arguments about God’s nature, because I am fine with “God is three and God is one”, and furthermore, I think that any precise attempt to explain that is going to be wrong in some serious way. (Although, no harm in trying, just remember that what you’ve got is a theory, not absolutely correct dogma. After years writing the Scholastic theological treatise Summa Theologica, Aquinas refused to complete it because he had an experience of God “I can write no more. I have seen things that make my writings like straw.”)

• Man is “a rational, mortal animal”. He incorporates the vegetable soul (as the Western medievals call it, the part that lives and incorporates the world into itself via food, which even plants do), the sensible soul (the part that receives senses from the outside, which animals do), and the rational soul. So Man is a unity of animal and rational soul (John calls the rational soul just “soul”, and is what is called “soul” in modern terms).
• Angels are also rational animals, but are immortal. I think angels are considered animals because they receive sense information, despite their lack of body.
• These souls, which John usually call substances (the Form of plant, animal, and thinking being) can be combined, the example being fire and iron being combined into hot iron. So if you cut with a hot knife, the knife substance/soul does the cutting and the fire substance/soul does the cauterizing.
• When two substances are combined in an individual, it is called a hypostasis. So an individual man is a hypostasis of an animal and a (rational) soul.
• Christ is a hypostasis of Man and God. (John makes a point of saying that Christ is therefore a hypostasis of three things: animal, soul, and divinity.) John calls the adding of divinity to Man in Christ deification.
• Christ did miracles with his divine nature and ate food with his human nature.
• Since we know from the Bible that God is one, all the ur-forms, like being, life, love must be united into God. So God is life itself, and being itself.
• God is also three: God the Father, God the begotten Son, and God the proceeding Holy Spirit. Jesus says that the Father is greater than he in the sense of more honorable in the same way that a father has greater honor than his child. But they are all the same substance (or nature, I was not clear which was which).
• Since God is unchanging, he does not suffer. I assume this is because suffering implies a non-suffering state, and changing from non-suffering to suffering would be change, and God is unchanging. Presumably, then, God would not rejoice, either?
• Christ experienced all of our natural passions (thirst, hunger, agony of torture, fear of losing existence, etc.). He did not have the fear resulting from mistrusting that God will not protect us, provide for us, etc. He also did not experience anger (at least, not the unjust kind that we have). John almost suggests that the devil uses our natural passions to tempt us to sin, but basically says that Christ controlled his natural passions. (So maybe God the Father does not suffer, but God experiences suffering [and presumably, joy] through God the Son?)
• The Virgin Mary is honored for the same reason we honor the places of Christ’s life, and the Cross on which he died, because Mary is the mother of God.
• Rational/thinking beings have free will (John is quite explicit on that point, multiple times). We know that rational beings have free will because both men and angels will be judged, and you cannot judge something unless it has free will—you cannot judge a choice unless there is actually a choice. Furthermore, love can only be given of free choice.
• Adam and Eve were created innocent, not perfect. The Tree of the Knowledge of Good and Evil was a choice, so that they could choose to obey God or not.
• When the Creed talks about Jesus descending into hell (sheol/hades, which was seen as a sort of shadowy, mindless existence), it means that he preached to the people in sheol and those who believed went up to heaven with him. (This is the “harrowing of Hell”, and is usually depicting on icons of the Resurrection.)
• The Eucharist is really the body and blood of Jesus, because, through the Holy Spirit, Christ’s body and blood are added to the physical elements, similar to how Christ’s divinity was added to the material flesh of Mary (probably now he would say to Mary’s egg cell, from which Jesus’ body presumably grew). John says that we should not inquire more precisely how the elements become Christ’s body and blood, but that “through the Holy Spirit” should suffice. (Note that this is very different from transsubstantiation, which says that the accidents of the bread and wine [taste, texture, etx.] remain the same, the form is changed from bread to Jesus’ body. Transubstatiation says that the elements change natures, while John says that the elements gain an additional nature.)
• Marriage is good, but virginity is better than good, because it is imitating the angels (who Hebrews says are superior to us)—and Christ. Because of our weakness, marriage was given to us, and is also necessary for humanity to continue. John dodges the question of what would happen if all of us could attain to the higher virtue; maybe God would just create new people?
• God did not create evil. In fact, evil is not even a thing, it is the absence or misuse of good. So evil comes about by our disobedience.
• God’s goal for Man is to participate in himself.
• (John did not talk about the nature of sin. He cites Maximus the Confessor several times, he does not talk about sin as moving away from God (being) towards non-being, although he comes very close.)

This is a nice summary of eastern Patristic theology, but I found that it generally does not come packaged in a way that is easily identifiable. So if you already know the idea exists, you can frequently find it in John, but if you do not know, it is likely to pass by unexamined, unless you are doing a close reading. For example, one of the Patristic beliefs is that God is being itself, presumably because there is this hierarchy of species (the Form of something), with their genus being the generalization of that, so everything that has being must have the ur-genus (not the technical Aristotelian term) of being, which is God. This is not spelled out. John does say that God is life itself, but kind of passes it by, so if all the substances and natures have numbed your mind, it is easy to pass it over.

Similarly, John does not talk at all about deification, which is central to Eastern Orthodox spiritual development, although I think they usually call it theosis. Judging from what John says about Christ, I assume that theosis is the process of union with Holy Spirit. In Christ, this presumably happened immediately (in fact, I think John says as much, although “and [Jesus] grew in favor with God and man”), but every Christian’s experience is that this is definitely a process with us. So perhaps theosis happens gradually for us, as our values, desires, and habits become conformed to those of Christ? John mentions our end state / God’s goal for us / our telos kind of in passing, so it is unclear if he thinks that we are going towards merely participating in God somehow or whether God is (slowly, painfully) untwisting us and gradually creating a Christlike hypostasis of Man + Holy Spirit (divinity). He does not mention how we are a new creation.

Protestant readers will disagree with John on various points, depending on the flavor of Protestant. It sounds like the Church Fathers saw God as emotionless, although it is hard for me to be sure, since John frequently adds a bunch of qualifications to statements about the nature of God that undo how the statement initially sounds. If “impassible” means that God does not experience emotions, I expect most Protestants are going to disagree. Charismatics may disagree that Christ did miracles with his divine nature—if so, how do we (or if you prefer, the saints) do miracles? The Charismatic view is that we do miracles as humans through partnering with Holy Spirit, so if Christ is fully human, would he not have done miracles the same way? Or do we gain the divine nature in some fashion through baptism, in which case, why do we frequently fail to see miracles when we command/ask/pray for them? Most Protestants will probably disagree with the Eucharist being actually Jesus’ body and blood, although that is what the Church has believed since the earliest recorded times. I am certainly a proponent of not taking Tradition unquestionably, but these differences are worth some thought. We should have a more solid reason for rejecting them than “my interpretation of the Bible says so, and because we are being reactionary against Roman Catholic excesses of the late Middle Ages”. John is synthesizing 700 years of Christian meditation here which is longer than even the longest Protestant tradition, Lutherans.

Histories always talk about how the rediscovery of Aristotle in the West in the 1100s was like a bomb going off, and now I can see why. The Aristotelian categories incite the mind—here is an elegant system that offers a way to unify the diversity of creation into a nice hierarchy. While I was reading this section I found myself stirred up about software development ideas that I have shelved because they are too big to realistically do. Not that software development is about categories, but it’s the same part of the brain, that focuses on united all the gnarly details into a elegant program. In the 1100s there were the Aristotelian embracers (the Averroists), and the deniers who clung to Augustine (the Augustinians), so it is easy to see how an Aquinas would create a systematic integration of the two. In this context, the Aquinine Scholasticism makes sense, and it makes sense why it would be adopted by the Catholic Church, since Plato has some obvious good points, while Aristotle also has some obvious good points, and they mitigate each other’s weaknesses. One could perhaps say that Plato focuses on unity (the One, the Forms), while Aristotle focuses on how to integrate diversity (genus → species → individual)

I think this book probably ]rewards revisiting topically. I found that reading it linearly—while something I advise—frequently caused my eyes to start sliding over the words, because the vocabulary is unfamiliar and often contrary to the modern meaning, and the philosophical assumptions are not mine at all. Reviewing the points in this summary revealed some things that I had missed on the first reading. I think revisiting this topically is useful, after having read it through once to get the high level sense of thought. The topical re-reading will naturally focus more closely on what he is saying, while the linear reading is likely to gloss over the details, unless maybe the reader is consciously focusing. Perhaps reading aloud would be helpful in avoiding eyes sliding over the text.