When the spirit of man has been made alive in Christ, it is through the Spirit of Christ in us. That is how we have part in Christ, through His Spirit when we have been born again. We had to be born again of Christ's Spirit because before new birth we were spiritually speaking "dead in trespasses and sins." When we have been born again, we have partaken of Christ's resurrection life (the first resurrection) and have overcome the second death.
I agree the Old Covenant saints are now a spiritual body of Christ in heaven without physical form, belonging to the "general assembly". They are now spiritually alive in heaven through the resurrection life of Christ. Before the advent of Christ, the Old Covenant faithful waited in death for Christ to come and rescue them from that place of dead from Old called Abraham's bosom. (Lu 16:22)
Ephesians 4:8-10 (KJV) Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men. (Now that he ascended, what is it but that he also descended first into the lower parts of the earth? He that descended is the same also that ascended up far above all heavens, that he might fill all things.)
These Old Covenant saints were in life faithful unto death and are now, since the advent of Christ part of the "innumerable multitude" of faithful saints that are the spiritual body of Christ in heaven after death. They are the 144,000 of all the tribes of the children of Israel from Old with the innumerable number of saints of "all nations, and kindreds, and people, and tongues." (Rev 7:9-17) John is shown here a vision of heaven as it shall be when the Kingdom of God in heaven is complete. The Old Covenant faithful saints were faithful unto death, and that's why John also sees them under the altar with the opening of the fifth seal. To be under the altar means they were covered by the sacrificial blood of the Lamb of God through promise until Christ literally came and made atonement for sin and defeated death. They could not ascend to heaven a spiritual body before the coming of Christ. But being faithful unto death they were reserved by God to be spiritually the firstfruits of God and the Lamb through Christ's Spirit. (Rev 14:1-5)
Revelation 6:9-11 (KJV) And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held: And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
The martyrs John saw as living souls without physical form in heaven are all who have already lived and died in faith during this period of time symbolized a thousand years. This is time that began with the first coming of Christ that shall not end until the seventh trumpet begins to sound that this particular time shall be no longer (Rev 10:5-7). We know a thousand years symbolizes time for proclaiming the gospel of the Kingdom of God because these martyred saints lived and died during this time.
Revelation 20:4 (KJV) And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
But these martyred souls do not show the full number of souls that shall be spiritually alive in heaven after death. Because John says there shall be others who are blessed and holy, have part in the first resurrection through Christ, and overcome the second death. They aren't numbered with the martyred, but they too shall live and reign with Christ during this time symbolized a thousand years.
Revelation 20:6 (KJV) Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.
Lastly in this chapter John sees a great white throne after the heaven and earth have fled away. John writes of the DEAD, not of faithful saints who live and die in faith during this time symbolized the thousand years. After seeing the fate of faithful saints in the Lord, John is also shown the fate of all who die in unbelief. In life they are never part of the spiritual body of Christ before they died. Therefore after time for becoming eternally saved through Christ has expired, the dead are judged according to what is found written in the books and the book of life and since in life they did not overcome the second death they shall be cast into the lake of fire.
Revelation 20:11-15 (KJV) And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them. And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works. And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works. And death and hell were cast into the lake of fire. This is the second death. And whosoever was not found written in the book of life was cast into the lake of fire.
