This expression tells us who the person are who are specially concerned in this book; and to whom the Revelation of Jesus Christ is shown. At the very outset we are thus warned that we are no longer on, but quite off, the ground of the Pauline Epistles, which are addressed to "sons," and not to "servants."
The word is (...), doulos, and means a bond servant.
Now, without denying that the members of the Body of Christ are in a certain sense the servants of Christ, yet it is also perfectly clear that this is not their title as to their standing in Christ before God. It is distinctly declared to each of them, "Thou art no more a servant, but a son" (Gal. iv.7). This is the one great point which is insisted on with reference to their new position in Christ.
Throughout the Old Testament, in passages too numerous to be counted, God's People Israel are constantly spoken of as His servants. This fact is too well known to need anything more than its bare statement.
Its significance will be at once seen when we come to the New Testament Scriptures. There we find the same use of the word whenever Israel is in question. It occurs 124 times; but as in 39 of these it refers to domestic servants, or those who serve man, we have to deal only with the 85 occurrences where it is used with reference to God. Of these 85, no less than 59 are in the Gospels and Acts. Only six in the Church Epistles (Rom. i.1; 1 Cor. vii.22; Gal. i.10; Eph. vi.6; Phil. i.1; Col. iv.12), and six in the general and other Epistles (2 Tim. ii.24; Tit. i.1; Jas. i.1; 1 Pet. ii.16; 2 Pet. i.1; Jude 1).
But while this is the case with the Epistles, the word "servants" occurs no less than fourteen times in the book of Revelation, and this, not in the exceptional manner, as in the Epistles, but as the one specific and proper title for those who are the subjects of the book.
In the Epistles the use is peculiar, as an examination of the passages will show. Out of the whole twelve, six are in the first verse of the Epistle,  describing the special character of the writer. For while all sons serve, and are in a sense, therefore, servants, yet "servants," as such, are not necessarily sons. In other words a "son" may be called a servant, but a "servant" can never be called a son.
Hence, the writers of the Epistles, being all engaged in special service, might well be called servants. And the Apocalypse, being written concerning Israel, the Israelites are, as appropriately, always spoken of as "servants."
This evidence may not seem conclusive in itself; but, taken with the other reasons given, it adds its cumulative testimony to our position that the book of Revelation has not the Church of God for its subject.
As the members of the Body of Christ, we are "in Christ." We have received a sonship-spirit, whereby we cry, Abba - i.e., my Father, "...and if children, then heirs, heirs of God, and joint-heirs with Christ" (Rom. viii.15-17).
"As many as are led by Divine-spirit (i.e., the new nature) are sons of God; for we have not received a bond-service spirit" (v.14,15). This is enlarged upon in Gal. iv.1-7, where the fact is still more clearly enforced and taught.
May we not ask why, if the Apocalypse be all about the Church of God, the people are never spoken of by this their new designation of "sons," but always by the title used of those in the Old Testament who were under the Law? Is it not passing strange that this should be so? And is it not the duty of those interpreters who see the Church as the subject of the book, to explain to us this striking peculiarity?
Even in the Gospels, in speaking to the Twelve, the Lord Jesus specially calls their and our attention to such a change in the relationship, which had then taken place. Not so great a change as that revealed and contained in the Mystery. He had been showing them somewhat of the future, and He says (John xv.15), "Henceforth I call you not servants; for the servant knoweth not what his lord doeth: but I have called you friends." In the Apocalypse, He is about to show them things which must come to pass hereafter; and He does not even call them "friends," still less does He speak of them as "sons," but He goes back and takes up still more distant ground, and calls them, without exception, "servants."
A careful study of the Old Testament with reference to this word "servants" will help to strengthen our position. In Lev. xxv.42, Jehovah declares of them, "they are my servants." Deut. is full of references to this great fact: and, when we pass to the Apocalypse, and read it as the continuation of God's dealings with Israel, then all is clear; and we have no problem to solve, as to why all is turned from light to darkness, and the "sons of God" are suddenly spoken of as "servants." Neither have we any difficulty to explain as to why those who are declared to be no more "servants," but "sons," are continually called servants, and not sons.
Even John himself, in writing by the same Spirit for the Church of God (1 John iii.2), when speaking of them, says, "Beloved, now are we the sons of God," and he calls them this in view of their seeing Him as He is, and their becoming like Him. But when he is writing for those who will be on the earth during the times of the Great Tribulation, he is Divinely inspired to speak of them, not as "the sons of God," but as the "servants of God."
We repeat once again, in order to make this point quite clear, that while "sons" may perform some special service, and therefore may, on that account, be called "servants:" "servants," on the contrary, whatever may be the service rendered, can never occupy the position, or have the title, of "sons."
 Rom., Phil., Tit., Jas., 2 Pet., and Jude.