For detail as to this “mark,” see on Revelation 13:16. That John here recalls this mark serves to draw attention to the pressure these saints experienced to betray their Lord and Saviour (despite the Deceiver’s imprisonment). They chose for their risen Lord in favour of the deceptive life-giving breath of the beast, a choice that cost them their lives. We are further to recall that the beast who had sought to force a mark on people’s foreheads and/or hands has been captured and thrown alive into the lake of fire (Revelation 19:20). It is only right and proper that those who resisted the now captured beast (at the cost of their lives) be vindicated.
4 Then I saw thrones, and seated on them were those to whom the authority to judge was committed. Also I saw the souls of those who had been beheaded for the testimony of Jesus and for the word of God, and those who had not worshiped the beast or its image and had not received its mark on their foreheads or their hands. They came to life and reigned with Christ for a thousand years.