One trying to understand salvation correctly should not begin with Revelations.
Whether one is 'saved always saved' is irrelevant.
Those who God has called to Jesus (no one comes to the Father but thru me John 14:6 No one can come to me unless the Father who sent me draws them John 6:44) and are faithful to the end of this age are the sons of God and will be part of the 1st res.
We'll need to go back a bit to set the scene for Rev 21:8
Blessed and holy are those who share in the first resurrection! The second death has no power over them, but they will be priests of God and of Christ, and will reign with Him for a thousand years. Rev 20:6
Who are they reigning over? Who are they priests to?
All the rest of humanity that is still alive after Jesus's return and are not raised to new life with true spirit life - as Jesus has. Obviously, 1000 years is a long time and humanity will continue to produce offspring and die. After the 1000 years, there is a second res.
The rest of the dead did not come back to life until the thousand years were complete. v5
Those living during the 1000 years and all those since Adam that ever lived (not including the prophets etc who have already been raised to new life) will be raised back to a physical/mortal life into a world of peace, truth and love made possible by the rule of Jesus and his priests etc. and the absence of the devil during that 1000 years without the constant barrage of evil, lies and deception.
He seized the dragon, that ancient serpent, who is the devil, or Satan, and bound him for a thousand years. v2
After that, he must be released for a brief period of time. v3
This is the time when all unsaved humanity will have to make a choice - for life/salvation in Christ, or if not, then there is no more life for them and they go to the second death - never to be seen ever again.
But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.” Rev 21:8
This is the second death to remove all those who choose to remain in their sin and have refused the grace offered in Jesus blood. This has nothing to do with the saved and the righteous in Christ. These have had their sin removed - covered in Jesus blood which is sufficient to cover all sin for all men - but all have to accept the offer God makes. No good works or any other means of salvation is possible.
Several misconceptions persist regarding the resurrections and who loves and who dies.
An irrational idea persists that the 1000 years is not, or cannot be literal - yet mentioned 6 times in the same passage! Several other erroneous ideas about the end of this age and the next are refuted here.