The object of this book is to sketch the development of the principal Christian doctrines from the close of the first century to the middle of the fifth. The choice of these frontiers is not so arbitrary or artificial as one might at first sight suppose. There is an obvious convenience in placing the starting-point outside the New Testament. Not only is its teaching a distinct, highly specialized field of study, but the difference of atmosphere becomes immediately apparent as one crosses from the apostolic to the post-apostolic age. At the other end the council of Chalcedon (451) saw the curtain begin to fall on the Church's first doctrinally creative period. Discussion was far from being closed; to take but one example, the Christological issue which Chalcedon had tried to settle continued as a subject of fierce controversy for generations. But, so far as the central stream of Christendom was concerned, the brilliant upsurge of fresh ideas which had distinguished the earlier centuries had spent itself. By the sixth century, both in East and West, the reign of formalism and scholasticism was well under way.

If he is to feel at home in the patristic age, the student needs to be equipped with at least an outline knowledge of Church history and patrology. Here there is only space to draw his attention to one or two of its more striking features. In the first place, he must not expect to find it characterized by that doctrinal homogeneity which he may have come across at other epochs. Being still at the formative stage, the theology of the early centuries exhibits the extremes of immaturity andsophistication. There is an extraordinary contrast, for example, between the versions of the Church's teaching given by the second-century Apostolic Fathers and by an accomplished fifthcentury theologian like Cyril of Alexandria. Further, conditions were favourable to the coexistence of a wide variety of opinions even on issues of prime importance. Modern students are sometimes surprised at the diversity of treatment accorded by even the later fathers to such a mystery as the Atonement; and it is a commonplace that certain fathers (Origen is the classic example) who were later adjudged heretics counted for orthodox in their lifetimes. The explanation is not that the early Church was indifferent to the distinction between orthodoxy and heresy. Rather it is that, while from the beginning the broad outline of revealed truth was respected as a sacrosanct inheritance from the apostles, its theological explication was to a large extent left unfettered. Only gradually, and even then in regard to comparatively few doctrines which became subjects of debate, did the tendency to insist upon precise definition and rigid uniformity assert itself.

Two important dividing-lines cut across the period, the one vertically and the other horizontally. The former is the difference of theological temperament between East and West. For historical reasons Rome and the churches immediately associated with her (Gaul, Spain, North Africa, etc.) developed in relative independence of the Eastern churches, and this is reflected in their creeds, liturgies and doctrinal attitude. While Greek theologians are usually intellectually adventurous and inclined to speculation, their Latin counterparts, with the exception of those subject to Eastern influences, seem by contrast cautious and pedestrian, confining themselves to expounding the traditional rule of faith. As an extreme example of this difference we need only juxtapose the conceptions of theology held by (a) Irenaeus and Tertullian, and (b) Clement and Origen, in the latter half of the second and first half of the third centuries. Deeply suspicious of, even hostile to, philosophy, the former limited the function of theology to expounding the doctrines set out in Holy Scripture; they applauded, the simple believers who were content with the rule of faith. The latter, on the other hand, went so far as to distinguish two types of Christianity, with two grades of Christians corresponding to there. The first and lower type was based on `faith', i.e. the literal acceptance of the truths declared in Scripture and the Church's teaching, while the second and higher type was described as `gnosis', i.e. an esoteric form of knowledge. This started with the Bible and tradition, indeed was founded on them, but its endeavour was to unravel their deeper meaning, and in the light of it to explore the profounder mysteries of God and His universe and scheme of salvation; it was supposed to culminate in mystical contemplation or ecstasy. Thus they divided the faithful into simple believers, whom they tended to disparage, and `spiritual' men, ' gnostics' or 'perfect', whom they regarded as specially privilcged by God.

The horizontal dividing line coincides with the reconciliation between Church and Empire effected by Constantine I (306-337), of which the council of Nicaea (325) was the symbol. Prior to this the Church was a persecuted body, struggling to adapt itself to its environment and to fight off such foes as Gnosticism. It is to its credit that, in spite of all difficulties, it was able to produce great constructive theologians like Irenacus and Origen. With the accession of Constantine, however, the situation radically changed. Henceforth, except for a brief in. terlude when Julian was sole emperor (361-3), the Church was to enjoy the often embarrassing favour of the State. The era of acute ecclesiastical controversy now began, and councils of bishops became the accepted instruments for defining dogma. As a matter of fact, Christian theology was now entering upon its first splendid summer, and the definitions hammered out against this background of controversy and often unedifying rivalries were to prove of lasting value. Because of the importance of this horizontal division, the material in this book has been arranged so as to take account of it.