When speaking to his disciples, Jesus said that some of them would not die before his return, and that those who had died under the law would be judged (the resurrection of Daniel 12) according to their works.

For the Son of man shall come in the glory of his Father with his angels; and then he shall reward every man ACCORDING TO HIS WORKS. Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
(Matthew 16:27 - 28)

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. (Revelation 20:13-13)

We are saved by grace through faith in Christ Jesus, not of works. "There is no condemnation for those who are in Christ."

Hell, Hades, Sheol... was a holding place for those who had died under the law, without a redeemer; those who were to be judged according to their works.

So when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory. O death, where is thy sting? O grave, where is thy victory? The sting of death is sin; and the strength of sin is the law. But thanks be to God, which giveth us the victory through our Lord Jesus Christ.
1 Corinthians 15:54 - 57

Death and hell were defeated at the cross.

"After the capture and overthrow of Jerusalem, as the prophets had foretold, and after the total destruction of the jewish nation, Titus, who had been appointed by the decree of God to avenge the blood of the Lord Jesus Christ, celebrated with his father Vespasian his victory by a triumph and closed the Temple of Janus."

- Orosius, Book 7

Jeremiah Lamenting the Destruction of Jerusalem, Rembrandt, 1630

THE WORLD WILL NEVER END

unto him be glory in the church by Christ Jesus throughout ALL AGES, WORLD WITHOUT END. Amen. Ephesians 3:21

But Israel shall be saved in the Lord with an everlasting salvation: ye shall not be ashamed nor confounded WORLD WITHOUT END. For thus saith the Lord that created the heavens; God himself that formed the earth and made it; he hath established it, he created it not in vain, he formed it to be inhabited. Isaiah 45:17 - 18

Of the INCREASE of his government and peace there shall be NO END, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even FOR EVER. Isaiah 9:7

But every man is tempted, when he is drawn away of his own lust, and enticed. Then when lust hath conceived, it bringeth forth sin; and sin, when it is finished, bringeth forth death. James 1:14-15

It's not the devil. It's not satan. It is your OWN lust that causes you to sin.

WHAT WAS THE MARK OF THE BEAST?

Was it an invisible sign of allegiance, a document, or currency (stamped with Caesar's image) that allowed citizens to buy and sell?

And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads: and that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name. Revelation 13:16-17

Roman citizens were required to publicly claim allegiance to Caesar by burning incense in his honor and proclaiming that “Caesar is Lord”. Those who did this received a document that allowed them to buy and sell in the marketplace. Without it, no one could purchase anything. Judeans were required to pay taxes to Caesar, and would eventually be required to offer sacrifices to him three times a year. The penalty for refusal was death. "We have no king but Caesar."

In the Old Testament, God marked His children on their hands and foreheads.

And it shall be for a sign unto thee upon thine hand, and for a memorial between thine eyes, that the Lord's law may be in thy mouth: for with a strong hand hath the Lord brought thee out of Egypt. Exodus 13:9

And it shall be for a token upon thine hand, and for frontlets between thine eyes: for by strength of hand the Lord brought us forth out of Egypt. Exodus 13:16

And these words, which I command thee this day, shall be in thine heart: And thou shalt bind them for a sign upon thine hand, and they shall be as frontlets between thine eyes. Deuteronomy 6:6 , 8

Therefore shall ye lay up these my words in your heart and in your soul, and bind them for a sign upon your hand, that they may be as frontlets between your eyes. Deuteronomy 11:18

Let's not forget that first century Christians were also sealed with an invisible mark on their foreheads.

And I heard the number of them which were sealed: and there were sealed a hundred and forty and four thousand of all the tribes of the children of Israel. Revelation 7:4 (See also: Revelation 14:1)

And the Lord said unto him, Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that be done in the midst thereof. Ezekiel 9:4

Some translations use the collective "man".

Wycliffe "Here is wisdom; he that hath understanding, acount the number of the beast; for it is the number of man, and his number is six hundred sixty and six."

Representing the sinful/apostate nature of man.

John 2:25 "and because he didn't need for anyone to testify concerning man; for he himself knew what was in man."