The explanatory material that fills out the remainder of the verse provokes varied interpretation. The reason for this is that two similar but variant readings of "was completed" have come down to us in the manuscripts: plerothosin and plerososin. The former is aorist passive, meaning, as in the New King James Version, that "the number of their fellow servants . . . was completed," while the latter is plain aorist, changing the sense to either "their fellow servants . . . should be complete" (less likely, according to the experts) or they "should complete [their course]" or "should fulfill [their calling]."
Yet, this may all be just a semantic argument. By using italics, most Bibles make it clear that the number of is not in the Greek text but has been supplied by the translators. This was done to conform to their misunderstanding of the passive form, plerothosin. Since mainstream Christians, including translators, do not believe in the biblical doctrine of sanctification as a lifelong process—in cooperation with God—of spiritual growth toward perfection, translations of this verse contain a built-in bias toward a certain number being saved by grace alone through faith rather than those whom God calls being transformed into the image of Christ through grace and works. Thus, they insert the italicized phrase unnecessarily to preclude the idea of Christian works—despite the fact that the entire passage exalts the particular works of witnessing and martyrdom!
Nevertheless, the verb—whichever is chosen as the better of the two—appears in the plural form, as it refers to its plural subjects, "servants and . . . brethren." "Number" is singular. This provides additional proof that Revelation 6:11 is not referring to a specific number of martyrs but simply that others either will be completed or will complete their calling through martyrdom. The latter half of the verse, then, is better rendered, ". . . until their fellow servants and their brethren, who are to be killed as they were, should also be complete [or, should also complete (their course)]." In other words, whether passively or actively, more sons and daughters will come to perfection through suffering and death, just as God's Firstborn Son did as our Forerunner (Hebrews 2:9-11).
Richard T. Ritenbaugh
The Fifth Seal (Part Two)