Jesus bore witness to the Father’s redeeming work (John 5:36). These martyrs embraced Jesus’ witness as it came to them through the gospel they received and ended up paying for it with their lives. In so doing they testified to the truth of this gospel. See also on Revelation 1:2.
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.