One can read the awkward sentence at the end of this passage in two different ways. Taken at face value, it seems to suggest that the future martyrs need to go through some sort of "completion" before their deaths. Most Bible translators, however, add a few words to fill out the picture. "Until the number of their fellow servants and brothers who were to be killed as they had been was completed." That is, the end will not come until history records a certain number of martyrs.
This was a popular idea in first-century Judaism. Statements similar to Revelation 6:9-11 occur in 1 Enoch 47:1-4 and 4 Ezra 4:35-37. The author of 4 Ezra wrote the book in reaction to the destruction of Jerusalem in A.D. 70, around the same time as the composition of Revelation. As a non-Christian Jew, the writer seeks to understand God's will and ways in the light of incomparable anguish and suffering.
Israel's history, from Old Testament times until A.D. 70, had more downs than ups. Magnificent promises and prophecies mingled with betrayals, apostasies, and disappointments. At the time of 4 Ezra's composition Jewish hopes for a national Israel seemed forever crushed. Thus "Ezra's" words ring through and through with sorrow.
In a "vision" Ezra wrestles with the vast number of faithful Jews who lost their lives in A.D. 70. How can God ever fulfill His promises when His own chosen people suffer such disasters? The angel Uriel responds that the whole current age is full of sadness and infirmity. Only in the age to come will the promises of God find their complete fulfillment.
"How long and when will these things be?" Ezra asks.
Uriel tells him, "Didn't the souls of the righteous in their chambers ask about these matters?" God's answer to them was: "When the number of those like yourselves is completed." Such an answer may not satisfy us today, but it expresses the idea that suffering does have a purpose, a limit, and an ultimate goal. We will never fully see justice in this world--only in the world to come.
Lord, I pray that I will not be distracted by my local and temporary perspective. Help me to trust in Your overall control of both the present and the future.