Loci theologici or loci communes , are the common topics of discussion in theology. As theology is the science which places in the light of reason the truths revealed by God, its topics are, strictly speaking, coextensive with the whole content of revelation. Usage, however, and circumstance have restricted the loci to narrower but ill-defined limits. Melanchthon, the theologian of Lutheran Germany, published in 1521 "Hypotyposes theologicæ seu loci communes", a presentation of the chief Christian doctrines drawn from the Bible as the only rule of faith. His avowed intention was to improve on similar works by John Damascene and Peter Lombard. Leaving aside undisputed dogmas which do not bear directly on the salvation of man, he expounds with scanty commentary, or none at all, the state of fallen man, free-will, sin, the law of God , the law of man, the Gospel, the power of the Law and the power of the Gospel, grace, justification, faith, hope, and charity, the difference between the Old and New Testament , the abolition of the Law through the Gospel, the sacraments of Baptism, Penance, and the Eucharist, authority, and scandal. Melanchthon's "Loci" became the textbook for Lutheran theology and the author has rightly been styled the prœceptor Germaniœ . Like Peter Lombard, he had his imitators and commentators, who formed a goodly body of Protestant Schoolmen. The greatest work of this kind is "Loci communes theologici", by John Gerard , professor at Jena, published in nine volumes (1610-1622); it is the greatest and also the last. After Gerard the loci theology gives place to systematic theology ; the unconnected exposition of "topics" in the light of the Bible gradually disappears. On the Catholic side Melanchthon's " Loci" were countered by the "Enchiridion locorum communium" of Johann Eck, which between 1525 and 1576 ran through forty-five editions. It was dedicated to Henry VIII of England. The topics which Eck expounds and defends against the Reformers are: the Church and her authority, the councils, the primacy of the Apostolic See, Holy Scripture , faith and works, confirmation, ordination, confession, communion under both kinds, matrimony, extreme unction, human laws feasts, fasts, the worship of saints and their images, the Mass, vows, clerical celibacy , cardinals and legates, excommunication, wars against the Turks, immunities and temporalities of the Church, indulgences, purgatory, annates, the burning of heretics, discussion with heretics, and infant baptism. Other Catholic writers followed on the track of the Ingolstadt professor; e.g. Franciscus Orantes (died 1584), Konrad Kling (died 1566), Joseph V. Zambaldi (died 1722), and Cardinal Bellarmine (q.v.), whose "Disputationes de controversiis fidei" (1581-92) are still the chief arsenal and stronghold of Catholic controversy. But, whilst Protestants concentrated their best theological effort on the loci , Catholics soon returned to the systematic methods of the older Summœ .
Cano (died 1560) applied the term loci theologici to a treatise on the fundamental principles or sources of theological science. On the threshold of every science there stands a complex of preliminary principles, postulates, and questions, which must be elucidated before progress is possible. Some are common to all sciences, some are peculiar to each. Before Cano the questions preliminary to theology had never been treated as a science apart, general dialectics being deemed a sufficient introduction. Cano observes that the "Queen of sciences " draws its arguments and proofs chiefly from authority, and only calls in reason as the handmaid of faith. Accordingly he sets up ten loci — sources of theology — without, however, pretending to limit them to that number. They are: the authority of Holy Scripture , of Catholic tradition, of general councils, of the Roman Church, of the Fathers, of the Schoolmen ; natural reason, the authority of philosophers and doctors in civil law, and the authority of history. The first seven are the proper places in which theology moves, the last three are useful auxiliaries. Melchior Cano's work gave a new turn to theological teaching. Much that before his time had been taken for granted, or, at best, only loosely investigated, became the favourite theme of the schools. The foundations of theology, which had lain embedded in the Christian mind, were laid bare, examined, strengthened, and rendered safe both for the believer inside the Church and against the foe without. The scientific method which takes nothing for granted, but investigates and probes to the very root every item of knowledge, is not a thing of yesterday, much less a child of anti-Catholic tendencies: Bishop Melchior Cano introduced it as the best weapon of offence and defence in religious warfare. The "Loci theologici" was first published in 1563, three years after the author's death, by the Grand Inquisitor Valdes. Twenty-six editions followed the first: eight in Spain, nine in Italy, seven in Germany, and two in France. Numerous writers during the following centuries produced works on the same lines: Seraphimus Ractius (Razzi) (died 1613), Petrus de Lorca (died 1606), Dominicus a S. Trinitate (died 1687), Ch. du Plessis d'Argentrée (died 1740), Franciscus Kranz, and many more. Gradually the subject-matter of the loci entered the body of theology under the title of "Prolegomena", general dogmatics, fundamental theology, or apologetics. In "A Manual of Catholic Theology", by Wilhelm and Scannell (London, 1906), the loci are treated in the first book under the following headings: the sources of theological knowledge ; Divine revelation ; transmission of revelation ; the Apostolic deposit of revelation ; ecclesiastical traditions; the rule of faith ; faith ; faith and understanding.
The necessity of meeting attacks on the Faith at the precise point on which they are directed has, of recent years, led to a modification in apologetic methods. Existing textbooks draw their proofs from Scripture, tradition and, when possible, from reason. The authority of these loci, or sources, having been previously proved, the demonstration is considered complete. But since evolutionism has taken hold of the modern mind and filled it with a never-satisfied desire to know the origin and the growth of all things in the realms of nature and of mind, the loci themselves have been submitted to fierce criticism by men who will be convinced by nothing but facts and experiments. They proceed by the positive, or historical, method which eliminates all supernatural factors, and retains only the bare facts linked together in an unbroken chain of causes and effects. The Bible to them is no longer the Word of God, but a mere collection of documents of various merit ; the Church is an institution of human origin. It must be confessed that the historical method is fraught with danger even to those who use it in defence of the Church. The danger is real but so is the necessity of facing it, for it is useless to argue from authority with men who acknowledge no authority. What is wanted is that the Catholic apologist keep a steady eye on the landmarks fixed by the Church, and deviate neither to the right nor to the left. With that precaution, the historical method is likely to become an abundant source of light and understanding on points of doctrine and discipline hitherto viewed out of their historical frame and in a borrowed light. Thus the discovery of the Didache has been a revelation which has upset many fond calculations, and the excavations in Palestine, Assyria, Egypt, and other places, where they bear on Bible history, have done more good than harm to the traditional views. The French are at the present day the pioneers of the historical treatment of dogma ; one need only point to the splendid series of "Studies in the History of Dogmas " published by Lecoffre in Paris.
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