To be thus faithful and just he needs a threefold
1. He must master the sources. For this purpose he must be acquainted with such auxiliary sciences as ecclesiastical philology (especially the Greek and Latin languages, in which most of the earliest documents are written), secular history, geography, and chronology. Then, in making use of the sources, he must thoroughly and impartially examine their genuineness and integrity, and the credibility and capacity of the witnesses. Thus only can he duly separate fact from fiction, truth from error.
The number of sources for general history is so large and increasing so rapidly, that it is, of course, impossible to read and digest them all in a short lifetime. Every historian rests on the shoulders of his predecessors. He must take some things on trust even after the most conscientious search, and avail himself of the invaluable aid of documentary collections and digests, ample indexes, and exhaustive monographs, where he cannot examine all the primary sources in detail. Only he should always carefully indicate his authorities and verify facts, dates, and quotations. A want of accuracy is fatal to the reputation of an historical work.
2. Then comes the composition. This is an art. It must not simply recount events, but reproduce the development of the church in living process. History is not a heap of skeletons, but an organism filled and ruled by a reasonable soul.
One of the greatest difficulties here lies in arranging the material. The best method is to combine judiciously the chronological and topical principles of division; presenting at once the succession of events and the several parallel (and, indeed, interwoven) departments of the history in due proportion. Accordingly, we first divide the whole history into periods, not arbitrary, but determined by the actual course of events; and then we present each of these periods in as many parallel sections or chapters as the material itself requires. As to the number of the periods and chapters, and as to the arrangement of the chapters, there are indeed conflicting opinions, and in the application of our principle, as in our whole representation, we can only make approaches to perfection. But the principle itself is, nevertheless, the only true one.
The ancient classical historians, and most of the English and French, generally present their subject in one homogeneous composition of successive books or chapters, without rubrical division. This method might seem to bring out better the living unity and variety of the history at every point. Yet it really does not. Language, unlike the pencil and the chisel, can exhibit only the succession in time, not the local concomitance. And then this method, rigidly pursued, never gives a complete view of any one subject, of doctrine, worship, or practical life. It constantly mixes the various topics, breaking off from one to bring up another, even by the most sudden transitions, till the alternation is exhausted. The German method of periodical and rubrical arrangement has great practical advantages for the student, in bringing to view the order of subjects as well as the order of time. But it should not be made a uniform and monotonous mechanism, as is done in the Magdeburg Centuries and many subsequent works. For, while history has its order, both of subject and of time, it is yet, like all life, full of variety. The period of the Reformation requires a very different arrangement from the middle age; and in modern history the rubrical division must be combined with and made subject to a division by confessions and countries, as the Roman Catholic, Lutheran, Reformed churches in Germany, France, England, and America.
The historian should aim then to reproduce both the unity and the variety of history, presenting the different topics in their separate completeness, without overlooking their organic connection. The scheme must not be arbitrarily made, and then pedantically applied, as a Procrustean framework, to the history; but it must be deduced from the history itself, and varied as the facts require.
Another difficulty even greater than the arrangement of the material consists in the combination of brevity and fulness. A general church history should give a complete view of the progress of Christ's kingdom in all its departments. But the material is so vast and constantly increasing, that the utmost condensation should be studied by a judicious selection of the salient points, which really make up the main body of history. There is no use in writing books unless they are read. But who has time in this busy age to weary through the forty folios of Baronius and his continuators, or the thirteen folios of Flacius, or the forty-five octaves of Schroeckh? The student of ecclesiastical history, it is true, wants not miniature pictures only (as in Hase's admirable compend), but full-length portraits. Yet much space may be gained by omitting the processes and unessential details, which may be left to monographs and special treatises. Brevity is a virtue in the historian, unless it makes him obscure and enigmatic. 
The historian, moreover, must make his work readable and interesting, without violating truth. Some parts of history are dull and wearisome; but, upon the whole, the truth of history is "stranger than fiction." It is God's own epos. It needs no embellishment. It speaks for itself if told with earnestness, vivacity, and freshness. Unfortunately, church historians, with very few exceptions, are behind the great secular historians in point of style, and represent the past as a dead corpse rather than as a living and working power of abiding interest. Hence church histories are so little read outside of professional circles.
3. Both scientific research and artistic representation must be guided by a sound moral and religious, that is, a truly Christian spirit. The secular historian should be filled with universal human sympathy, the church historian with universal Christian sympathy. The motto of the former is: "Homo sum, nihil humani a me alienum puto;" the motto of the latter: "Christianus sum, nihil Christiani a me alienum puto."
The historian must first lay aside all prejudice and party zeal, and proceed in the pure love of truth. Not that he must become a tabula rasa. No man is able, or should attempt, to cast off the educational influences which have made him what he is. But the historian of the church of Christ must in every thing be as true as possible to the objective fact, "sine ira et studio;" do justice to every person and event; and stand in the centre of Christianity, whence he may see all points in the circumference, all individual persons and events, all confessions, denominations, and sects, in their true relations to each other and to the glorious whole. The famous threefold test of catholic truth -- universality of time (semper), place (ubique), and number (ab omnibus) -- in its literal sense, is indeed untrue and inapplicable. Nevertheless, there is a common Christianity in the Church, as well as a common humanity in the world, which no Christian can disregard with impunity. Christ is the divine harmony of all the discordant human creeds and sects. It is the duty and the privilege of the historian to trace the image of Christ in the various physiognomies of his disciples, and to act as a mediator between the different sections of his kingdom.
Then he must be in thorough sympathy with his subject, and enthusiastically devoted thereto. As no one can interpret a poet without poetic feeling and taste, or a philosopher without speculative talent, so no one can rightly comprehend and exhibit the history of Christianity without a Christian spirit. An unbeliever could produce only a repulsive caricature, or at best a lifeless statue. The higher the historian stands on Christian ground, the larger is his horizon, and the more full and clear his view of single regions below, and of their mutual bearings. Even error can be fairly seen only from the position of truth. "Verum est index sui et falsi." Christianity is the absolute truth, which, like the sun, both reveals itself and enlightens all that is dark. Church history, like the Bible, is its own best interpreter.
So far as the historian combines these three qualifications, he fulfils his office. In this life we can, of course, only distantly approach perfection in this or in any other branch of study. Absolute success would require infallibility; and this is denied to mortal man. It is the exclusive privilege of the Divine mind to see the end from the beginning, and to view events from all sides and in all their bearings; while the human mind can only take up things consecutively and view them partially or in fragments.
The full solution of the mysteries of history is reserved for that heavenly state, when we shall see no longer through a gloss darkly, but face to face, and shall survey the developments of time from the heights of eternity. What St. Augustine so aptly says of the mutual relation of the Old and New Testament, "Novum Testamentum in Vetere latet, Vetus in Novo patet," may be applied also to the relation of this world and the world to come. The history of the church militant is but a type and a prophecy of the triumphant kingdom of God in heaven -- a prophecy which will be perfectly understood only in the light of its fulfilment.
 The German poet, Friedrich Rückert, thus admirably enjoins the duty of condensation: Wie die Welt läuft immer weiter, Wird stets die Geschicte breiter Und uns wird je mehr je länger Nöthig ein Zusammendränger: Nicht der aus dem Schutt der Zeiten Wühle mehr Erbärmlichkeiten, Sondern der den Plunder sichte Und zum Bau die Steine schichte Nicht das Einzle unterdrückend Noch damit willkühlich schmückend, Sondern in des Einzlen Hülle Legend allgemeine Fülle; Der gelesen Alles habe, Und besitze Dichtergabe, Klar zu schildern mir das Wesen, Der ich nicht ein Wort gelesen. Sagt mir nichts von Resultaten! Denn die will ich selber ziehen. Lasst Begebenheiten, Thaten, Heiden, rasch vorüberziehen."