John had earlier used this phrase in his letter to Smyrna (Revelation 2:11). Death involves the separation of the soul from the body (for the sake of clarity let us call this the “first” death). The “second death” involves the separation of people from God and so is descriptive of being cast into the lake of fire (Revelation 20:14; Revelation 21:8). The present passage promises that this lake has neither hold nor claim on these martyrs. The verdict rendered to them by those seated on the thrones is permanent.
6 Blessed and holy is the one who shares in the first resurrection! Over such the second death has no power, but they will be priests of God and of Christ, and they will reign with him for a thousand years.