This poignant cry has often been raised by suffering believers anxious to see the Lord work on their behalf. It was often recorded in Scripture under such circumstances, first in Psalm 6:3 and last of all in our text above (see also Psalm 74:10; 90:13; Zechariah 1:12; etc.).
The cry in our text is from "the souls of them that were slain for the word of God, and for the testimony which they held" (Revelation 6:9). The context would indicate that these souls are of those who will die under future persecutions, but the principle surely applies to the martyrs of every age. When they were slain, it was only their physical bodies which died, while their souls live on in heaven, aware of events on earth. This truth, in fact, applies to all who die trusting in Christ as their Savior.
All of these men and women, both while yet alive in the body and also later when "absent from the body, and . . . present with the Lord" (2 Corinthians 5:8), have longed for the day when Christ will come to complete "the redemption of the purchased possession" (Ephesians 1:14) and when finally God will answer the age-long prayers of His people as they prayed, "Thy will be done in earth, as it is in heaven" (Matthew 6:10).
For several generations now, Christians have been asking (even singing!) the great question of our text, "How long, O Lord?"
The signs of His soon coming multiply, yet we still wait. Nevertheless, He has promised to return, and His Word is true and sure. He will come--perhaps today! "For yet a little while, and he that shall come will come, and will not tarry" (Hebrews 10:37). HMM