We find, in fact, that every Epistle meets some particular difficulty and danger. No letter of Paul can be understood without the admission of the actual imperfection of his congregations. He found it necessary to warn them even against the vulgar sins of the flesh as well as against the refined sins of the spirit. He cheerfully and thankfully commended their virtues, and as frankly and fearlessly condemned their errors and vices.
The same is true of the churches addressed in the Catholic Epistles, and in the Revelation of John. 
The seven Epistles in the second and third chapters of the Apocalypse give us a glimpse of the church in its light and shade in the last stage of the apostolic age -- primarily in Asia Minor, but through it also in other lands. These letters are all very much alike in their plan, and present a beautiful order, which has been well pointed out by Bengel. They contain (1) a command of Christ to write to the "angel" of the congregation. (2) A designation of Jesus by some imposing title, which generally refers to his majestic appearance (Rev.1:13 sqq.), and serves as the basis and warrant of the subsequent promises and threatenings. (3) The address to the angel, or the responsible head of the congregation, be it a single bishop or the college of pastors and teachers. The angels are, at all events, the representatives of the people committed to their charge, and what was said to them applies at the same time to the churches. This address, or the epistle proper, consists always of (a) a short sketch of the present moral condition of the congregation -- both its virtues and defects -- with commendation or censure as the case may be; (b) an exhortation either to repentance or to faithfulness and patience, according to the prevailing character of the church addressed; (c) a promise to him who overcomes, together with the admonition: "He that hath an ear, let him hear what the Spirit saith unto the churches," or the same in the reverse order, as in the first three epistles. This latter variation divides the seven churches into two groups, one comprising the first three, the other the remaining four, just as the seven seals, the seven trumpets, and the seven vials are divided. The ever-recurring admonition: "He that hath an ear," etc., consists of ten words. This is no unmeaning play, but an application of the Old Testament system of symbolical numbers, in which three was the symbol of the Godhead; four of the world or humanity; the indivisible number seven, the sum of three and four (as also twelve, their product), the symbol of the indissoluble covenant between God and man; and ten (seven and three), the round number, the symbol of fulness and completion.
As to their moral and religious condition, the churches and the representatives fall, according to the Epistles, into three classes:
1. Those which were predominantly good and pure, viz., those of Smyrna and Philadelphia. Hence, in the messages to these two churches we find no exhortation to repentance in the strict sense of the word, but only an encouragement to be steadfast, patient, and joyful under suffering.
The church of Smyrna (a very ancient, still flourishing commercial city in Ionia, beautifully located on the bay of Smyrna) was externally poor and persecuted, and had still greater tribulation in view, but is cheered with the prospect of the crown of life. It was in the second century ruled by Polycarp, a pupil of John, and a faithful martyr.
Philadelphia (a city built by king Attalus Philadelphus, and named after him, now Ala-Schär), in the province of Lydia, a rich wine region, but subject to earthquakes, was the seat of a church likewise poor and small outwardly, but very faithful and spiritually flourishing -- a church which was to have all the tribulations and hostility it met with on earth abundantly rewarded in heaven.
2. Churches which were in a predominantly evil and critical condition, viz., those of Sardis and Laodicea. Here accordingly we find severe censure and earnest exhortation to repentance.
The church at Sardis (till the time of Croesus the flourishing capital of the Lydian empire, but now a miserable hamlet of shepherds) had indeed the name and outward form of Christianity, but not its inward power of faith and life. Hence it was on the brink of spiritual death. Yet Rev.3:4 sq., distinguishes from the corrupt mass a few souls which had kept their walk undefiled, without, however, breaking away from the congregation as separatists, and setting up an opposition sect for themselves.
The church of Laodicea (a wealthy commercial city of Phrygia, not far from Colosse and Hierapolis, where now stands only a desolate village by the name of Eski-Hissar) proudly fancied itself spiritually rich and faultless, but was in truth poor and blind and naked, and in that most dangerous state of indifference and lukewarmness from which it is more difficult to return to the former decision and ardor, than it was to pass at first from the natural coldness to faith. Hence the fearful threatening: "I will spew thee out of my mouth." (Lukewarm water produces vomiting.) Yet even the Laodiceans are not driven to despair. The Lord, in love, knocks at their door and promises them, on condition of thorough repentance, a part in the marriage-supper of the lamb (3:20).
3. Churches of amixed character, viz., those of Ephesus, Pergamum, and Thyatira. In these cases commendation and censure, promise and threatening are united.
Ephesus, then the metropolis of the Asian church, had withstood, indeed, the Gnostic errorists predicted by Paul, and faithfully maintained the purity of the doctrine delivered to it; but it had lost the ardor of its first love, and it is, therefore, earnestly exhorted to repent. It thus represents to us that state of dead, petrified orthodoxy, into which various churches oftentimes fall. Zeal for pure doctrine is, indeed, of the highest importance, but worthless without living piety and active love. The Epistle to the angel of the church of Ephesus is peculiarly applicable to the later Greek church as a whole.
Pergamum in Mysia (the northernmost of these seven cities, formerly the residence of the kings of Asia of the Attalian dynasty, and renowned for its large library of 200,000 volumes and the manufacture of parchment; hence the name charta Pergamena; -- now Bergamo, a village inhabited by Turks, Greeks, and Armenians) was the seat of a church, which under trying circumstances had shown great fidelity, but tolerated in her bosom those who held dangerous Gnostic errors. For this want of rigid discipline she also is called on to repent.
The church of Thyatira (a flourishing manufacturing and commercial city in Lydia, on the site of which now stands a considerable Turkish town called Ak-Hissar, or "the White Castle," with nine mosques and one Greek church) was very favorably distinguished for self-denying, active love and patience, but was likewise too indulgent towards errors which corrupted Christianity with heathen principles and practices.
The last two churches, especially that of Thyatira, form thus the exact counterpart to that of Ephesus, and are the representatives of a zealous practical piety in union with theoretical latitudinarianism. As doctrine always has more or less influence on practice, this also is a dangerous state. That church alone is truly sound and flourishing in which purity of doctrine and purity of life, theoretical orthodoxy and practical piety are harmoniously united and promote one another.
With good reason have theologians in all ages regarded these, seven churches of Asia Minor as a miniature of the whole Christian church. "There is no condition, good, bad, or mixed, of which these epistles do not present a sample, and for which they do not give suitable and wholesome direction." Here, as everywhere, the word of God and the history of the apostolic church evince their applicability to all times and circumstances, and their inexhaustible fulness of instruction, warning, and encouragement for all states and stages of religious life.
 The remainder of this paragraph is taken in part from my Hist. of the Apost. Church (108, pp. 427 sqq.), where it is connected with the life and labors of St. John. Comp. also the monographs of Trench and Plumptre on the Seven Churches, and Lange's Com. on Rev. 2 and 3.