Read the bible -it actually makes sense -people make stuff up about it that doesn't make sense.
The first to be cast into gehenna are the beast and false prophet -at about the return of Christ.....
Rev 19:20 And the beast was taken, and with him the false prophet that wrought miracles before him, with which he deceived them that had received the mark of the beast, and them that worshipped his image. These both were cast alive into a lake of fire burning with brimstone.
A thousand years later, the devil -who will have been bound in the bottomless pit for the thousand years -will be loosed for a short time -and only then be cast in to gehenna.....
Rev 20:6 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.
Rev 20:7 And when the thousand years are expired, Satan shall be loosed out of his prison,.....................
Rev 20:10 And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever.
Those who were "in Christ" at his return -the dead first, and then the living -will be made immortal at his return -and reign on earth with him for the thousand years -after which the "rest of the dead" will be raised to the judgment....
1Th 4:15 For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep.
1Th 4:16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
1Th 4:17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
Rev 5:10 And hast made us unto our God kings and priests: and we shall reign on the earth.
Rev 20:4 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.
Rev 20:5 But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection.
Rev 20:6 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.
The end of verse 20:5 does not describe the beginning of that verse -but what came before it.
Then comes the "great white throne" judgment.... where all who have ever lived will be judged "according to their works" -not their innocently-errant beliefs or ignorance. God is just, equal and merciful -such things will be considered, and not all who are raised to the second resurrection/the judgment will be cast into the lake of fire.
Rev 20:11 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.
Rev 20:12 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.
Rev 20:13 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.
Rev 20:14 And death and hell were cast into the lake of fire. This is the second death.
Rev 20:15 And whosoever was not found written in the book of life was cast into the lake of fire.
Dan 12:1 And at that time shall Michael stand up, the great prince which standeth for the children of thy people: and there shall be a time of trouble, such as never was since there was a nation even to that same time: and at that time thy people shall be delivered, every one that shall be found written in the book.
Dan 12:2 And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.
Though God is able to completely destroy any in the lake of fire -this is not necessarily what will happen to all who are cast there.
Mat 10:28 And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
Rev 21:8 But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.
1Co 3:13 Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is.
1Co 3:14 If any man's work abide which he hath built thereupon, he shall receive a reward.
1Co 3:15 If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire.
1Co 15:21 For since by man came death, by man came also the resurrection of the dead.
1Co 15:22 For as in Adam all die, even so in Christ shall all be made alive.
1Co 15:23 But every man in his own order: Christ the firstfruits; afterward they that are Christ's at his coming.
1Co 15:24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power.
1Co 15:25 For he must reign, till he hath put all enemies under his feet.
1Co 15:26 The last enemy that shall be destroyed is death.
Notice that death and the grave are cast into the lake of fire...
Rev 20:14 And death and hell were cast into the lake of fire...............
If any are completely destroyed in the lake of fire, they will not be the sort you would miss. Any who go into it and come out of it will be better for it.
Mal 3:2 But who may abide the day of his coming? and who shall stand when he appeareth? for he is like a refiner's fire...........