Christian writers naturally turned to the Bible for their teaching on the nature of God. But their use of it was often influenced by the philosophical thought of their own day. The Hebrews, we saw in Chapter 9, pictured the God whom they worshipped as having a body and mind like our own, though transcending humanity in the splendour of his appearance, in his power, his wisdom, and the constancy of his care for his creatures. Such a conception, set out in the earlier books of the Old Testament, retained its authority despite some later changes of emphasis. But this biblical view, we noted on p. 58, was radically modified in the teaching of Philo of Alexandria. Philo, a devout Jew, does indeed insist on God's moral attributes: his patience, his wisdom and his loving care. But he also presents him as the metaphysical first principal of the universe, without bodily form or human passions, indeed without any sensible qualities: a perfectly simple, unchangeable, unfathomable being, who can only be positively described in the words of Exodus 3:14 as ‘He who Is’.

Christian writers developed a broadly similar view, partly because they were influenced by the same philosophical authorities, partly through direct imitation of Philo himself. To this they added their doctrine of the Trinity, which will concern us later; here too their thinking was influenced by Philo, especially in his conception of the divine Logos; but there was less in the way of actual borrowing.

Plato's most important pupils were his nephew Speusippus, who succeeded him as head of the Academy on his death in 347; Xenocrates, who followed Speusippus, 339–314; and Aristotle, who broke away from Plato's influence and after spending a period away from Athens returned in 335 to found a school of his own, the Lyceum. Meanwhile he had acted as tutor to the young Alexander the Great. On Alexander's death in 323 an anti-Macedonian movement in Athens induced him to leave the city, and he died the next year.

Aristotle's influence on Western thought can hardly be exaggerated; but he was not a major influence on Christianity during its first four centuries, and for that reason will be rather briefly treated here. Indeed the scope and originality of his thought were not generally appreciated for several centuries after his death. The reasons for this change of fortune can be found in the history of his writings.

The works which were accessible to early Christians and other non-specialist enquirers were in the main those which he called his ‘exoteric’ or popular writings, which though lacking Plato's literary genius were carefully written to appeal to the general reader. These works have not survived, though to some extent they can be reconstructed from surviving fragments. It appears that they were written early in life, while Plato's influence was still powerful, and that they take an idealistic view of the aims of philosophy, besides showing signs of personal religious feeling.

During some four centuries in late antiquity, from the second till early in the fifth, two systems of belief and moral direction existed side by side. At the start of this period well-educated men in civilized Europe looked to philosophy for guidance; as we have seen, the Platonic tradition was already strong and would soon be dominant. Philosophy was taken to comprise logic, ethics and physics, which included the beginnings of what we now call natural science. Ancient logic led up to the theory of knowledge; ethics enquired what sorts of good we should aim at securing, and how to achieve them in practice.

At the outset of this period Christianity did not look like a counterpart to philosophy; indeed it was not always recognized as a distinct movement independent of the Judaism from which it sprang. But it developed very rapidly, and by the end of our period it had captured the intellectual allegiance of cultivated citizens in both the Eastern and the Western Empire. Compared with other religions of its time and place it was far more successful in organizing its beliefs into a coherent system. In doing this it borrowed largely from philosophy, and especially from Platonism. But it kept a sharply defined identity; its commitment to the Bible as a sacred book was far more uncompromizing than the philosophers' respect for Plato; and it valued communal experience and tradition in a way which offended students accustomed to accepting the guidance of expert scholars. Nevertheless philosophy helped to mould its beliefs about God and the world, and taught it to uphold them in argument.

Augustine is by far the ablest philosopher of late antiquity. He is also one of its best-known characters; he reveals his inmost thoughts at many points in his voluminous writings, but particularly in the Confessions, which recalls his early life and his conversion, and in the Retractations, one of his last productions, in which he gives a chronological survey of his written work and explains the corrections which he now wishes to establish. More than most ancient writers, his thought underwent a development which is closely related to his changing occupations and concerns. This makes it difficult to capture in a brief account; all its phases are interesting and worthy of note. And there is a particular difficulty in treating of Augustine as a philosopher. The enormous literature he has inspired of course includes examinations of his philosophical thought; but the great majority of these are written by admirers, who moreover share the basic assumptions of his metaphysics. Many modern philosophers, at least in the Anglo-Saxon tradition, reject these assumptions altogether, or in large part, as I am impelled to do myself, and are thus bound to be handicapped in providing a survey which is at once appreciative, scholarly and critical.

Augustine was born in 354 at Tagaste in North Africa, about 200 km west of Carthage and some 700 km south-west of Rome. His father Patricius was a pagan till late in life, his mother Monica a Christian.

Christian theology begins with the New Testament. The earliest Christians, it shows us, were ready to expound the Scriptures and to defend their faith in Jesus against Jewish and pagan opponents. But before long they faced the new task of expressing their beliefs in a way that well-educated pagans could understand and appreciate. And as the Christian movement expanded, new expressions of the faith were devised, and had to be examined and approved so that as far as possible disagreements and misconceptions could be avoided. The great development of early Christian theology took place in the three hundred years extending from the mid-second century to the Council of Chalcedon in 451: the age of Irenaeus, Origen, Athanasius and Augustine.

At this time the various schools of philosophy gave their adherents many of the benefits we now expect from religion. The conventional state religion was often little more than a formality; the so-called ‘mystery cults’ offered comfort and reassurance, but provided no explanations and made few demands; the worshipper could enter one, two or several such fellowships as he wished. The Jews had largely detached themselves from the main stream of ancient culture. It was the philosophers who both called for commitment and presented a way of life based on a rational view of the world and man's place within it.

Philosophy was invented and given to the world by the Greeks. Although in some departments they drew on the experience of other nations (for instance, on Babylonian astronomy), it was the Greeks who developed philosophy into a wide complex of studies, which included the beginnings of what we now call natural science, and which was later to be summed up in the three headings of logic, ethics and physics. Physics was the name given to the study of the natural world and its explanatory principles; it therefore took in the question whether there are gods, or a single God, and whether the world was made, and is governed, by such beings. For those who believed in divine existence, theology was a branch of physics.

The Greek philosophers broke new ground through their ability to ask abstract and wide-ranging questions. Before their time, much common-sense observation was embodied in the working knowledge of sailors, farmers and builders, or expressed in proverbial sayings about human conduct. But for the large general questions about the world men had to resort to a primitive mythology which associated each of the main components of the world with a particular divine being; the heavens with Zeus, the sea with Poseidon, and so on. A philosophy recognizably distinct from mythology began when the sages of Miletus, in Asia Minor, attempted to explain the world in terms of inanimate things which could be expected to behave in a regular way in accordance with a few simple laws.

Plato is probably the greatest of the Greek philosophers; without question he made the greatest contribution to Christian theology. Not that he himself set out to expound a system or doctrine; his genius lay rather in raising profound and far-reaching questions in an informal style with the minimum of technical terms. To some of these questions he gave definite answers; in many cases he was content to demonstrate the complexity of a problem and the considerations to be borne in mind, partly as an exercise in rational discussion, but mainly from a deeply serious conviction of the difficulty of attaining the whole truth, and a dislike of premature solutions. The later Platonist philosophers seldom imitated this open, undogmatic approach, but made selections from his writings which seemed relatively consistent and could be defended against opposing schools. Among Christians an open, uninhibited approach to philosophy was revived for a time, especially under the influence of the Alexandrians Clement and Origen, in the third and fourth centuries, when bold speculations could be excused as ‘exercises’, gymnasiai; and in this period the influence of Platonist writers made itself felt on Christian theology. But this in turn developed a fixed dogmatic outline, reinforced by the authoritative decisions of Church councils. From that time on most Christians quoted Plato solely where he appeared to confirm established doctrines of the Church; the reality of God, his creation and providence, the heavenly powers, the human soul, its training, survival and future judgement, could all be upheld by appropriate choice of Platonic texts.

We are accustomed to sum up the Christian doctrine of the Trinity in the Latin formula derived from Tertullian, three Persons in one substance. Greek theologians confess three hypostases but one ousia. The last two words, whatever their precise meaning, are clearly authorized by the Nicene Creed – both the original version of 325 and the customary version attributed to the Council of 381 – which states that the Son is homoousios with the Father; the word can be rendered in Latin form either as ‘consubstantial’ or as ‘coessential’. But problems arise from the fact that the Latin word essentia, which is the exact equivalent of ousia, fell out of use. Latin theologians therefore translated ousia by substantia which is etymologically equivalent to the Greek ‘hypostasis’. The Latins thus used substantia to express the divine unity; the Greeks used the corresponding word to confess three hypostases: Father, Son and Spirit. Consequently the Greeks were liable to suppose that the Latin una substantia (like ‘one hypostasis’) denies the Trinity. The Latins, conversely, could suspect the Greeks of preaching ‘three substances’, three distinct or different Godheads. Nor was this merely a difference of technical terms; both sides were to some extent influenced by their own terminology, so that at important moments the Latins were inclined to put their main emphasis on divine unity, the Greeks on a clearly articulated Trinity.

The word homoousios has been much discussed; but the traditional explanations are unreliable. We begin by considering its constituent parts. First, the prefix homo- suggests unity or togetherness; but its sense is not perfectly constant.

The previous chapter has alluded to problems which arose from an imperfect understanding of the terms ousia and hypostasis. In the West Tertullian's formulation una substantia, tres personae was accepted by thinkers whose theological interests were perceptibly different from his own; whereas Tertullian was anxious to uphold the distinct existence of all three Persons, his Roman contemporaries and successors laid more stress on the divine unity. In the East there was no agreed formula, and tensions were sharper. Many Easterners reacted strongly against modalist teaching. Origen spoke of three hypostases, partly in order to maintain the substantial reality of Son and Spirit; but the phrase came to suggest his distinctive trinitarian teaching which ranked the three Persons in a descending scale of dignity and power. Some Easterners, either to avoid this implication or in imitation of Western usage, spoke of ‘one hypostasis’; but this again could suggest that only the Father is substantially real, the others being only his energies or functions. In the heat of theological debate there was little concern for reflection on the terms employed. Theologians drew no clear distinction between hypostasis and ousia, though there was some difference of nuance. Origen's use of the latter term was not clear-cut. Later Origenists thus disliked the term homoousios, which seemed uncomfortably close to the much-detested ‘one hypostasis’; but they were not unalterably opposed to it; a hard-line insistence on three ousiai remained a rarity.

In a review-article in IPQ, (1989) of O'Connell's book on The Origin of the Soul in St Augustine's Later Works I was puzzled as to why, if Augustine's final position on the origin of the human soul is neither traducianist (the soul as well as the body is handed down from one's parents) nor creationist (individual souls are constantly being created by God), these two rejected hypotheses are still apparently on the table until the end of Augustine's life (e.g. Incomplete Work 2.177–181). O'Connell thinks not only that Augustine was unprepared to be too dogmatic, but that as a wary ecclesiastical campaigner he was concerned not to give a handle to dangerous opponents.

Augustine at least ‘knows’ that our souls all shared Adam's sin, so that what he expresses ignorance about is not ‘our’ existence ‘in Adam’, but only how, after existing in Adam, our souls arrived in our present bodies. Yet it is still strange that creationism is not rejected outright, for it implies the making of new (and therefore guiltless) souls. Now it is true, as O'Connell regularly notes, that creationism was a very popular Christian belief outside North Africa. (Both Jerome and Pelagius subscribed to it.) So it may be that, since Augustine can find no knock-down scriptural text or reasoned argument against it, his profession of ignorance merely reflects an unwillingness to condemn a position so widely and traditionally held. Compare his unwillingness to condemn women who had committed suicide to avoid rape, because he knew they had been honoured in the Church as martyrs.

Augustine knows (and tells Jerome in Letter 166, for example) that squaring creationism with original sin is peculiarly difficult.

In The Master (2.3) Augustine quotes the following line from Virgil:

When Augustine's son Adeodatus is asked how many signs the line contains, he replies Eight – surprisingly if his father is supposed to be availing himself of Stoic theory. Thus he seems to assume that each individual word is a sign but that the verse as a whole, or the proposition [if] ρ, does not signify or refer to anything further, that is, beyond what is signified by each of its parts. Nevertheless, commentators on The Master often assume that for Augustine sentences do have some signification beyond that of the individual words of which they are composed, and that Augustine follows, but does not state, let alone defend, a theory to that effect. In fact the commentators are right, but they do not know why they are right.

The theory derives from Porphyry, and Augustine probably acquired his familiarity with it from one of Victorinus' commentaries, either that on Aristotle's Categories or that on Cicero's Topics. It may be briefly explained as follows: when Porphyry decided, contrary to the view of his master Plotinus, that Aristotelian logic could be put to the service of Platonic metaphysics, he argued precisely that a proposition about the ‘ordinary’ world, which, prescinding from Platonic metaphysics, appears to make sense, consists of a subject plus a concept (noema, or ennoia) which indicates the special and disambiguating features of the subject in each case. (If the concept is a second-order word (like a universal), it may be called an epinoia.)