Let's bear in mind that a Greek word can have several English meanings and it is the literary and historical context that tells us what its meaning is.
Rev 21:6 Then he said to me, "It is done! I am the Alpha and the Omega, the beginning and the end. To the thirsty I will give water as a gift from the spring of the water of life.
Rev 21:7 Those who conquer will inherit these things, and I will be their God and they will be my children.
Rev 21:8 But as for the cowardly, the faithless, the polluted, the murderers, the fornicators, the sorcerers, the idolaters, and all liars, their place will be in the lake that burns with fire and sulfur, which is the second death."
Second death. The Greek for 'second' is very, very clear. You can't argue around it. Therefore, 'death' cannot be talking about a physical death. You cannot physically die twice (medically, you can, but not physically as the Bible uses it- they had no notion of being 'medically dead'). So death:
G2288
θάνατος
thanatos
Thayer Definition:
1) the death of the body
1a) that separation (whether natural or violent) of the soul and the body by which the life on earth is ended
1b) with the implied idea of future misery in hell
1b1) the power of death
1c) since the nether world, the abode of the dead, was conceived as being very dark, it is equivalent to the region of thickest darkness, i.e. figuratively, a region enveloped in the darkness of ignorance and sin
2) metaphorically, the loss of that life which alone is worthy of the name
2a) the misery of the soul arising from sin, which begins on earth but lasts and increases after the death of the body in hell
3) the miserable state of the wicked dead in hell
4) in the widest sense, death comprising all the miseries arising from sin, as well physical death as the loss of a life consecrated to God and blessed in him on earth, to be followed by wretchedness in hell
Since it cannot be 1, it must be 2, 3, or 4. Any way you slice this passage, it is not talking about everyone going to heaven- certainly not immediately. They must at least spend some time in misery after they die from their sins. But wait. The passage also talks about the lake of fire. That phrase is used elsewhere.
Rev 20:12 And I saw the dead, great and small, standing before the throne, and books were opened. Also another book was opened, the book of life. And the dead were judged according to their works, as recorded in the books.
Rev 20:13 And the sea gave up the dead that were in it, Death and Hades gave up the dead that were in them, and all were judged according to what they had done.
Rev 20:14 Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire;
Rev 20:15 and anyone whose name was not found written in the book of life was thrown into the lake of fire.
You again cannot get around this verse either. Anyone's name not found in the book of life goes to the lake of fire. Whether you interpret this literally or metaphorically, the meaning is clear. Some people don't make it at the end, in the final judgment. But there's that lake of fire reference again.
Rev 20:10 And the devil who had deceived them was thrown into the lake of fire and sulfur, where the beast and the false prophet were, and they will be tormented day and night forever and ever.
The lake of fire is a place where people are tormented day and night 'forever and ever'. Greek:
G165
αἰών
aiōn
Thayer Definition:
1) for ever, an unbroken age, perpetuity of time, eternity
2) the worlds, universe
3) period of time, age
Part of Speech: noun masculine
The word is used twice. Even if you wanted to use 3, it would read 'for ages and ages'. 2 Would not make sense. Those who enter the lake of fire are tormented for ages and ages, forever.
So let's go back to Matthew 25, which there seems to be some confusion about:
Mat 25:46 "TheseG3778 will goG565 awayG565 into eternalG166 punishmentG2851, but the righteousG1342 into eternalG166 lifeG2222."
G166
αἰώνιος
aiōnios
Thayer Definition:
1) without beginning and end, that which always has been and always will be
2) without beginning
3) without end, never to cease, everlasting
Part of Speech: adjective
The Greek is very clear. It cannot be 1 or 2, therefore it must be 3. Those who do not meet the standard go to a punishment that has no end. The same word is then used in verse 41:
Mat 25:41 "ThenG5119 He will alsoG2532 sayG3004 to thoseG3588 on His leftG2176, 'DepartG4198 from Me, accursedG2672 ones, into the eternalG166 fireG4442 which has been preparedG2090 for the devilG1228 and his angelsG32a;
Again, the lake of fire, and again, it has no end.