Jesus Christ was the first to rise out of the dead. Jesus was, literally, the "first resurrection." This fact, well attested by the writings of the New Testament, forms the basis for understanding Revelation 20:5-6:

"This is the first resurrection. Blessed and holy is the one who has a part in the first resurrection; over these the second death has no power" (Revelation 20:5-6)

The first resurrection was Jesus Christ:

Revelation 1:5
Jesus Christ, the faithful witness, the first-born out of the dead

Acts 26:23
Christ should suffer and...be the first that should rise from the dead

Colossians 1:18
He is the beginning, the firstborn from the dead

1 Corinthians 15:20
Christ hath risen out of the dead--the first-fruits of those sleeping he became

Jesus Christ was, plainly, the first resurrection. This fact forms the basis of St. John's depiction of the tribulation martyr saints becoming full partakers of the "first resurrection" in Revelation 20--everything Christ received by his death and resurrection is granted to them. Revelation 20:4-6, therefore, depicts the reality of Pauline theology concerning the identity Christ's followers had "in Him." Paul had taught that the saints were to become partakers of Christ's own reign and victory over death. Paul, with his detailed theology of our baptism into the very death and resurrection of Jesus (Rom 6:3-14), taught that the saints had co-resurrection and co-enthronement in the realized resurrection and enthronement of Jesus Christ:

The saints have co-resurrection in Jesus Christ:

Ephesians 1:20 (Christ's resurrection)
He raised Him from the dead and seated Him at His right hand in the heavenly places

Ephesians 2:5-6 (the saints' co-resurrection in Christ)
[God] made us alive together with Christ...and raised us up with Him

Colossians 2:12 (the saints' co-resurrection in Christ)
ye are risen with him through the faith of the operation of God, who hath raised him from the dead

Colossians 3:1 (the saints' co-resurrection in Christ)
If ye, then, be risen with Christ , seek those things that are above

The saints have co-enthronement in Jesus Christ

Ephesians 1:20-22 (Christ's enthronement)
He raised Him from the dead and seated Him at His right hand in the heavenly places, far above all rule and authority and power and dominion, and every name that is named, not only in this age but also in the one to come. And He put all things in subjection under His feet

Ephesians 2:6 (the saints' co-enthronement in Jesus Christ)
God...raised us up with Him, and seated us with Him in the heavenly places in Christ Jesus

Colossians 3:1 (the saints' co-enthronement in Jesus Christ)
Therefore if you have been raised up with Christ, keep seeking the things above, where Christ is, seated at the right hand of God.

The suffering, overcomer saints reign with Him

Revelation 3:21 (cf. 2:26-27)
To him that overcometh will I grant to sit with me in my throne

2 Timothy 2:11-12
If we be dead with him, we shall live with him; if we suffer, we shall also reign with him

Philippians 3:8-12
I have suffered the loss of all things, and count them but rubbish so that I may gain Christ, and may be found in Him...that I may know Him and the power of His resurrection and the fellowship of His sufferings, being conformed to His death in order that I may attain unto the resurrection of the dead. Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that

The tribulation saints of Revelation 6:9-11, after being told to rest for a short time that the rest of the shed blood of the appointed martyrs might fill up the fullness of God's wrath (Matt 23:34-36; Rev 18:20,24; 19:2; 16:4-6,19; 17:6; 18:5-6; 1 Thess 2:14-16), were granted the realization of their co-inheritance with Christ--this realization of their hope is depicted in Revelation 20:4-6. St. John shows us the tribulation saints co-raised and co-enthroned with Jesus, realizing the full hope of St. Paul's teaching on the saints' reign and resurrection in the reign and resurrection of Jesus.

In sum, Revelation 20:4-6 is a narrative depiction of the saints' realization of the glorious promise Paul held out for them in his teachings--the saints are depicted as having attained the goal for which they all strove. As Paul taught, their resurrection and reign was "in Christ," and their sufferings and martyrdoms were honored by God with the reward of partaking in Christ's own resurrection, enthronement, and reign. They realized the promise of Paul's teaching that the saints were truly to take part in the first resurrection, the resurrection of Jesus Christ. Truly, on these the second death has no power (Rev 20:6).