The fifth seal opened
Rev 6:9 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
The fifth seal is opened by Jesus, for only Jesus was able to open the seals. This time it is a heavenly view, under the altar of God, of those saints killed for their testimony, the testimony of Jesus Christ, the gospel message. They were martyrs for their faith in our Lord. God would reward them and avenge them.
Rev 6:10 And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?
What of those who kill, thinking they are serving god? They were serving the wrong god, not the God of Abraham, Isaac and Israel. Their god makes promises, but he is a liar. Apart from Jesus is judgment for works and hell.
Rev 6:11 And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
Those in heaven were given white robes, the righteousness of Jesus Christ, as fellow heirs in glory. There were more to come, the faithful witnesses in the time of tribulation. Will we be as faithful if we face tribulation and persecution? Be ready, we are not promised freedom from persecution, but we are promised that God's wrath will not come upon us. Who knows, but that many may be saved by our faithful witness.