Early Christian Thought.
Early Christians regarded Greek religion as holding views unworthy of God, but they were divided as to Greek philosophy. Christian philosopher Justin Martyr (c. 100-c. 165) saw Christianity as compatible with the highest and best Greek thought, whereas Tertullian (c. 160-c. 225) dismissed philosophy, saying that Jerusalem (faith) could have nothing to do with Athens (philosophy).
Having been born out of Judaism, Christianity was unambiguously monotheistic and affirmed that God created the material of the universe out of nothing (ex nihilo). But it also affirmed the Trinity as multiplicity within unity, a view it regarded as implicit in Judaism.
Consistent with theism, Augustine (354-430) regarded God as omniscient, omnipotent, omnipresent, morally good, the creator (ex nihilo) and sustainer of the universe. Despite these multiple descriptors, God is uniquely simple. Being entirely free, he did not have to create, but did so as an act of love. As his creation, it reflects his mind. Time and space began at creation, and everything in creation is good. Evil is uncreated, being a lack of good and without positive existence. Though God is not responsible for evil even it has a purpose: to show forth what is good, especially what is good within God. Augustine developed a theme found as early as Plato, Aristotle, and Zeno of Citium, that God is a perfect being. After enumerating a hierarchy of excellencies (things to be "preferred") Augustine affirms that God "lives in the highest sense" and is "the most powerful, most righteous, most beautiful, most good, most blessed" (On the Trinity, XV, 4). When we think of God, we "attempt to conceive something than which nothing more excellent or sublime exists" (Christian Doctrine, I, 7, 7). But where Aristotle concluded that the greatest being must be aware only of himself, Augustine emphasized an opposite and distinctly Christian theme: God loves creatures supremely to the point of becoming incarnate in Christ in order to be revealed to them and to reconcile them to himself. Moreover, God is providentially active in history, from an individual level (Confessions) on up to dealings with entire nations (City of God). So as to the important subject of God's relationship to the world, Christian thought could not be more opposite Aristotle's view of a Being who contemplates only himself.
John Scotus Erigena (c. 810-c.877) had stronger affinities for Neo-Platonic thought. God created the universe according to eternal patterns in his mind and it is an expression of his thought, however incomplete an expression the cosmos may be. Erigena's pantheistic tendencies can be seen in his notion that God creates out of himself and "God is in all things." Creation is not in time but is eternal. In the process God used universals and made them particulars (e.g., humanity became individual persons). Immortality is the reverse process of particulars going back to universals. In Erigena's terms, division is the process of differentiating universals into particulars; analysis is the reverse, a return to unity and thus to God. These are not mere mental activities but mirror reality and God's relationship to the world. God is ultimately unknowable, being beyond all language and categories. Aristotle's predicates and categories cannot apply to God because they assume some type of substance. Nevertheless God can be described, albeit inadequately, using both positive and negative statements. Positive statements are only approximate but can be made more exact by adding negative statements. For example, it can be said that God is good (positive), but also that he is not good (negative) in that he is above goodness. These can be combined in the statement that he is "supergood." In spite of these approximations, God must be reached by mystical experience.
Islamic Neoplatonist al-Farabi (875-950) held that universals are in things and have no existence apart from particulars. Objects are contingent in that they may or may not exist; they do not have to exist. Therefore there must be something that has to exist-that exists necessarily-to ground the existence of all other (contingent) things. This being is God. The world evolves by emanation, and matter is a phase of that process. The potential in matter is made actual, and over time God brings out its form. Thought is one emanation from God, and through it knowledge arises in humans. The actualized human intellect becomes an immortal substance.
Avicenna (Ibn Sina; 980-1037), a Muslim, also distinguished between God as the one necessary being and all other things, which are contingent. The world is an emanation from God as the outworking of his self-knowledge. As such it is eternal and necessary. God must be eternal and simple, existing without multiplicity. In their essence, things do not contain anything that accounts for their existence. They are hierarchically arranged such that the existence of each thing is accounted for by something ontologically higher. At the top is the one being whose existence is necessary. From contingent things we come to know universals, whereas God knows universals prior to their existence in things.
Al-Ghazali (1058-1111) challenged any joining of theology and philosophy, holding that because the mind and senses are subject to error, truth must come by divine grace. Rather than the world existing necessarily in a Neoplatonic sense, it exists by the will of God alone. It is in no way autonomous, and even causal relationships are non-necessary. He rejected as un-Islamic Avicenna's view that things like souls or intellects could be eternal.
Anselm (1033-1109), archbishop of Canterbury, raised the perfect being concept to a new level by making it the foundation of his celebrated ontological argument. He accepted that God is the highest level of being under which there are, by degrees, lesser and lesser beings. Similar to Plato, Anselm assumes the realist view that entities which share an attribution, such as "good," also share in being. And somewhere there must be a perfection of that being (e.g., perfect goodness). That perfection is God.
Though a Muslim and an Aristotelian, Averroes (Ibn Rushd; 1126-1198) added to the growing concept of emanation by claiming that the universal mind is an emanation from God. Humans participate in this universal mind and only it, not the soul, is immortal. The mind of the common person understands religious symbols in a literal way, whereas the philosopher interprets them allegorically. Consequently, something understood as true philosophically may be untrue theologically, and vice versa.
Working from Judaism, Maimonides (1135-1204) accepted creation rather than an eternal universe. He drew from philosophic traditions to formulate three proofs based on the nature of God, and these were developed further by Aquinas. Following Aristotle Maimonides demonstrated the existence of a Prime Mover, and with some inspiration from Avicenna, the existence of a necessary being. He also showed God to be a primary cause. Though he considered God's existence demonstrable, he held that nothing positive could be said about God.
Bonaventura (John of Fidanza, c. 1221-1274) argued that the Aristotlean denial of Platonic ideas would entail that God knows himself but not the world. As such God could not be its creator. Furthermore, because some change in the universe is cyclic and therefore unexplainable by chance, change would have to be deterministic. But this would deny God's providence as well as human moral responsibility. So a proper concept of God must include Platonic ideas. Reason can prove God as creator since an eternal universe entails both that the amount of time of its existence is infinite and that it is increasing. Yet there cannot be both an infinite and a larger infinite (a view not held in modern times).
Thomas Aquinas (1225-1274) accepted both Aristotle and Christian revelation. He accepted both reason and revelation as sources of knowledge of God. Over the neo-Platonic notion of a hierarchy of reality in which lower existences are less real and a mere shadow of the divine, Aquinas accepted gradations of form and matter. Atop the hierarchy is God as pure form and no matter. As pure actuality and no potentiality, he is perfect and therefore changeless. He is also pure intelligence and pure activity. To these Aristotelian concepts Aquinas added Christian convictions that God is loving, providential, and ruler of the universe. Reason and revelation are in harmony because they have the same divine source, and revelation is not unreasonable. Perception is also in harmony because the world's origins are divine. This being the case, God as cause can be known through the world as effect. For this reason empirical facts ground Aquinas's theistic proofs.
The God that can be known in part from the universe is fundamentally different from it. Only God is identical to his essence, being neither more nor less than it. By contrast, a being such as Socrates is transcended by humanity because there are other people. On the other hand, Socrates has qualities ("accidents") that are not part of his essence; for example, he may be sitting. So unlike God, Socrates is both greater than and less than his essence. There is nothing that transcends God so nothing is greater than his essence. And there are no accidents in God because accidents are caused by something else (just as part of the cause of Socrates sitting is a chair).
God is not (completely) knowable because he is not material, whereas our knowledge is normally dependent on our senses. Furthermore, we normally know things by knowing their genus and species, yet God is unique and so cannot be known in that way. We can know something of God the negative way (via negativa) by removing limits, concluding for example, that God is unmoved, and unlimited by space. What we can know of God positively is neither exactly like our knowledge of temporal things (univocal) nor entirely different (equivocal). Rather, it is analogical, being in some ways the same and in other ways different. God knows x in a way that is both like and unlike the way in which Socrates knows x. God knows, but in a way that is, among other things, complete, immediate, and timeless.
That God created is evident (though not provable) because a material universe cannot emanate from an immaterial being. The universe exists to manifest God, who created the fullest possible range of beings because in them he can be revealed to the fullest extent. Beings range from angels, who are immaterial; to humans, who are material and immaterial; to animals, who are purely material (and both eat and move); to plants, to inanimate objects.
God as primary cause works through such created things as secondary causes. Nevertheless, creatures with a will remain free and responsible. God can also work apart from secondary causes in what we call miracles. Being good, God created the best possible world in the sense that it has the best kinds of things. Evil is a privation or lack of good and as such God did not cause it the way he causes other things. So we cannot ask why God brought about evil, but we can ask why he did not bring about more good. He did not bring about more good in order that he could be revealed through the greatest range of things, and as well, to allow for certain types of good (such as compassion, which can exist only where there is some suffering).
Aquinas and others grounded the scholastic synthesis of knowledge in the view that truth, morality, and God himself could be known by reason because the divine will itself is guided by reason. What is reasonable is therefore what is true and right. But John Duns Scotus (1265-1308) claimed that in humans and in God it is the will--not the intellect--that is primary. Evidence of this is that a being must will what to think about, thus something must act on the intellect; whereas nothing need act on the will. The view entails that there is no reason why God acts or wills as he does. This makes truth and morality essentially arbitrary and thereby unknowable through reason. God could have willed different moral standards. Scotus's view makes our knowledge of God a matter of revelation and faith, not of reason.
Another concept about God's will further destabilized the medieval world view. William of Ockham (1285-1347) held that omnipotence means God can do literally anything. Accordingly, a person could perceive something by sheer act of divine will, without the object being there at all. On his view, faith and reason can be contradictory. Ockham's "razor" sought to cut from explanations those entities that are unverifiable thereby making simpler explanations preferred. This was later used to cut out of world views such things as divine purposes, which had been central to explanations since the Greeks. Eventually, even concepts of a divine being would be optional--or even unnecessary--to explanations and world views.
The connection between reason and God was further undermined by Meister Eckhart's (1260-1327/28) view that God is "above being" and that human unity with the divine must be suprarational. Knowledge is a matter of proceeding from particulars to unity, beyond which is a unity with the divine surpassing all differences, "a silent desert." The divine being is therefore inexpressible. God knows all things in their unity, timelessly; but on our temporal level it makes sense to differentiate time as well as events.