The first period is that which contains the great question, almost the greatest which Ecclesiastical History has to answer, -How was the transition effected from the age of the Apostles, from Christianity as we see it in the New Testament, to Christianity as we see it in the next century, and as, to a certain extent, we have seen it ever since?
No other change equally momentous has ever since affected its fortunes, yet none has ever been so silent and secret. The stream, in that most critical moment of its passage from the everlasting hills to the plain below, is lost to our view at the very point where we are most anxious to watch it; we may hear its struggles under the overarching rocks; we may catch its spray on the boughs that overlap its course; but the torrent itself we see not, or see only by imperfect glimpses. It is not so much a period for Ecclesiastical History as for ecclesiastical controversy and conjecture. A fragment here, an allegory there; romances of unknown authorship; a handful of letters of which the genuineness of every portion is contested inch by inch; the summary examination of a Roman magistrate; the pleadings of two or three Christian apologists; customs and opinions in the very act of change; last but not least, the faded paintings, the broken sculptures, the rude epitaphs in the darkness of the catacombs, -these are the scanty, though attractive, materials out of which the likeness of the early Church must be reproduced, as it was working its way, in the literal sense of the word, 'under ground,' under camp and palace, under senate and forum, -'as unknown, yet well known; as dying, and behold it lives.'
from pages 30- 31, History of the Eastern Church