Why eschatological univeralism?

Here's why I am an eschatological universalist (postmillennial + idealist + eschatological universalist):

Why idealism?

Revelation is a book of symbols. Simply put, placing Revelation's timeline to try to fit and match the signs here and there with historic events leads to baffling inconsistencies in my estimation.
Is the beast:
Nero? (preterism)
Islam? (historicism)
The Pope/Papacy? (historicism, reformed confessions)
One particular man who appears to have power over death, but is in fact an impostor antichrist? (futurism)

Or does it make more sense to take the beast symbolically like we do the rest of Revelation? In which case, the "beast" need not refer to a particular person or office, but particular types of persons and offices (making the beast a typology, not one particular man). The office of antichrist may belong to those who unlawfully claim lordship over God's creation, which would indeed include people like the above mentioned; but I have the advantage because typologies are not historical. I don't need to work to fit all the puzzle pieces together to identify the beast, who appears in different moments of the symbolic timeline of Revelation. The other advantage? I can take much more scripture literally than the other camps, so long as I let symbolic books remain symbolic and not try to apply a historical hermeneutic to them to make sense.

This also allows me to make sense of the "little season" mentioned so often in scripture. Heaven is timeless -- little seasons cannot be timeless, for else they could not be little. Therefore, little season refers to those on the earth, where they must pass through their "little season". That also means that Satan's binding in Revelation 20:3 is not referring to his disability to deceive the nations. Satan is unbound on the earth for a little season, which little season is referring to the history of the church age. What does that mean for the gospel? I'll explain that later.

Revelation clearly describes a symbolic 1000-year period that transpires in heaven. Revelation 20:4-6 (note that this is immediately after Satan is "released" for a little while): "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. The rest of the dead did not come to life until the thousand years were ended. This is the first resurrection. Blessed and holy is the one who shares in the 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."

Those John saw in the vision are those saints who had passed through the "little season" -- they lived on the earth, then died and then went up into the first resurrection -- to reign with Christ. Christ's reign is from heaven. He is seated by the Father's right hand (1 Corinthians 15:25 "For he must reign until he has put all his enemies under his feet") and from there he reigns with the saints, where indeed Satan IS bound.

Why postmillennialism?

We are told that Christ must reign until all enemies have been made his footstool: Psalm 110:1, Isaiah 66:1, Luke 20:43, Acts 2:35, Acts 7:49, Hebrews 1:13, Hebrews 10:13, 1 Corinthians 15:27, Ephesians 1:22 Hebrews 2:8.

Where is Christ sitting? Look back at 1 Corinthians 15:

But in fact Christ has been raised from the dead, the firstfruits of those who have fallen asleep. For as by a man came death, by a man has come also the resurrection of the dead. For as in Adam all die, so also in Christ shall all be made alive. But each in his own order: Christ the firstfruits, then at his coming those who belong to Christ. Then comes the end, when he delivers the kingdom to God the Father after destroying every rule and every authority and power. For he must reign until he has put all his enemies under his feet. The last enemy to be destroyed is death.

(1 Corinthians 15:20-26 ESV)

Where is Christ sitting? At the Father's right hand,
How long does Christ sit there? UNTIL all his enemies are made his footstool.
When will Christ no longer sit there? When he destroys ALL his enemies. The LAST enemied to be destroyed is death.
What happens when Christ destroys death: Christ delivers the kingdom to God the Father.

Christ cannot sit at the Father's right hand if he returns before the millennium, during which time people will still die and death has not been destroyed.

Any amillennialist and postmillennialist will agree here. So why "postmil" instead of "amil"?

While the above is in agreement with both the post- and a- camps, I take "postmillennialism" to refer to the general state of the creation: the restoration of the whole world. Amillennialism tends to be mildly pessimistic about the end results of most men. Most amils are inclined to believe that the majority of mankind will end up in hell. Postmils believe that the majority of mankind will be saved.

Why do I believe that the majority of mankind will be saved?
There is no prayer of Jesus that goes unanswered, and the answer is always "yes" (2 Cor 1:18-22), therefore:
The Lord's Prayer will be fulfilled. (Matthew 6)
The Great Commission is now being and will be fulfilled (Matthew 28:18-20) -- "make disciples of all nations"
Jesus has been given the nations as his heritage (Psalm 2:7-9)
The kingdoms of this world will become the kingdom of our God (Rev 11:15)
The blood of the unrighteous are symbolically numbered in Revelation 14:20. -- This limitation (1600 stadia) is not set upon those who are righteous in Christ.
Ezekiel 37 describes an "exceedingly great army", something which has never been attributed to unbelievers. Too many to count, we are left with the descriptor "exceedingly great army".
Abraham's offspring, rightly understood, was blessed with multiplication, and we are inheritors of that promise, being grafted into Israel through Christ. How many? "as the stars of heaven and as the sand that is on the seashore" (Genesis 22:15-19)
Bringing things back to the millennium, during which time, the gospel sees success, people turn to the Lord, and learn from his ways, and his law. The word of the Lord judges between the nations, and the result of this? War ends. Swords are beaten into plowshares, and spears into pruning hooks, and nations no longer learn war. This is a peaceful period, in the "latter days", which describes the millennium (Isaiah 2:2-5, Joel 3:10, Micah 4:3).
There will be a period of time, when the infants do not die young, and old men do fill out their days, and those who die at the age of 100 will be considered "young", and yet at the same time, unrighteous sinners and unbelievers will experience this effect as well, though they will be "accursed". Our labor won't be in vain, our children will not be borne for calamity, and those who experience this will be the "offspring of the blessed of the Lord, and their descendants with them". Even better? "The wolf and lamb shall graze together; the lion shall eat straw like the ox, and dust shall be the serpent's food". -- this describes the millennial period, during which time the lifespan of humanity in general is increased, and mankind flourishes because of the blessing of the Lord, yet people still die, because we are not in resurrected bodies (Isaiah 65) -- This will be an untold period of prosperity and peace for the church and the whole earth, because "they shall not hurt or destroy in all my holy mountain, says the Lord".
-- ad nauseum.

Why eschatological universalism:

Jesus returns to a saved world (all enemies are under his feet) to deliver it to the Father. (1 Cor 15:25-26)
The final apostasy (an amillennial notion) requires that Satan be "bound" for the gospel to have success, and that when Satan is "unbound" to "deceive the nations", the gospel will then fail. The Great Commission fails. This makes the success of the gospel contingent on the binding and loosing of Satan, not on the power of the Holy Spirit. In other words, the gospel cannot succeed where Satan is loosed -- rather than the gospel succeeding even though Satan IS loosed! -- which is why the theologian B.B. Warfield said thusly (read at https://www.monergism.com/millennium-and-apocalypse):

"Of course the passage (xx. 1-10) does not give us a direct description of "the intermediate state." We must bear in mind that the book we are reading is written in symbols and gives us a direct description of nothing that it sets before us, but always a direct description only of the symbol by which it is represented. In the preceding vision (xix. 11-21) we had no direct description of the triumph and progress of the Gospel, but only of a fierce and gruesome war: the single phrase that spoke of the slaying sword as "proceeding out of the mouth" of the conqueror alone indicated that it was a conquest by means of persuading words. So here we are not to expect a direct description of the "intermediate state": were such a description given, that would be evidence enough that the intermediate state was not intended, but was rather the symbol of something else. The single hint that it is of the condition of the "souls" of those who have died in Christ and for Christ that the seer is speaking, is enough here to direct our thoughts in the right direction. What is described, or rather, to speak more exactly - for it is a course of events that is brought before us - what is narrated to us is the chaining of Satan "that he should deceive the nations no more"; the consequent security and glory of Christ's hitherto persecuted people; and the subsequent destruction of Satan. It is a description in the form of a narrative: the element of time and chronological succession belongs to the symbol, not to the thing symbolized. The "binding of Satan" is, therefore, in reality, not for a season, but with reference to a sphere; and his "loosing" again is not after a period but in another sphere: it is not subsequence but exteriority that is suggested. There is, indeed, no literal "binding of Satan" to be thought of at all: what happens, happens not to Satan but to the saints, and is only represented as happening to Satan for the purposes of the symbolical picture. What actually happens is that the saints described are removed from the sphere of Satan's assaults. The saints described are free from all access of Satan - he is bound with respect to them: outside of their charmed circle his horrid work goes on. This is indicated, indeed, in the very employment of the two symbols "a thousand years" and "a little time." A "thousand years" is the symbol of heavenly completeness and blessedness; the "little time" of earthly turmoil and evil. Those in the "thousand years" are safe from Satan's assaults: those outside the thousand years are still enduring his attacks. And therefore he, though with respect to those in the thousand years bound, is not destroyed; and the vision accordingly requires to close with an account of his complete destruction, and of course this also must needs be presented in the narrative form of a release of Satan, the gathering of his hosts and their destruction from above."

In other words, the "millennium" in question refer to believers in heaven, and therefore, to take the "unbinding" of Satan and apply it to the nations of the "world" is contra-contextual to the rest of the vision, especially in the preceding context in which Revelation is referring to the "rest" of believers in the heavely state.

This, of course, denies the notion of the apostasy by referring Revelation 20's "binding" and "loosing" back to the context in which John wrote it. And if there is no "final apostasy" that occurs literally prior to the second advent, then we can take the above verses for postmillennialism to their literal and consistent meaning -- when Christ returns to the earth, he returns to a saved earth.