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Paul never wrote anything about beheadings, but he ...
Right, because Paul was NOT writing about OUR end of era scenario.
(Paul) did identify the beast out of the earth in 2 Thessalonians 2:1-12 ...
No Paul does not identify the Beast in 2 Thess or anywhere else in his letters.
You must be hallucinating. Get help.
and was given greater detail by John in Rev 13 and 17.
Not just "given greater detail. Rev 13 & 17 contain the ONLY descriptions of the Beast.
ONLY Revelation 13-17 reveal how to identify the Beast. Which is a reason
Satan wants people to look everywhere else in regard to end time information.
What is written in Rev 20:4 is the vision John was given beginning with John the Baptist who was the first one to be beheaded for the witness of Christ and everyone thereafter until the last day when Christ returns with His army of angels from heaven, Rev 19: 11-21.
What many do not know is that in all reality the mark of the beast has always been beginning with those who followed and bowed down to Nimrod who established the Babylonian Empire by the River Euphrates, Genesis 10:8-10. The Targum says, Nimrod became a mighty man of sin, a murderer of innocent men and a rebel before the Lord.
Nimrod was also a false Messiah who married his own mother, Semiramis, and created his own evil religious system called Baal and worshiped and bowed down to their sun god.
The revelations are not something new under the sun in John's day when he received them, but began with Nimrod and his false religion that still remains today with the seven remaining nations that Daniel describes in Daniel chapter 7. So yes, many have taught about the beast and its mark and many have been beheaded for the witness of Christ beginning with John the Baptist.
You say "The revelations are not something new under the sun in John's day when he received them, ..."
Wrong. The God sent Revelation was very "new".
There has NEVER before been a "marking" system like the Revelation predicts ....
nor a WORLD RULER like the Revelation predicts.
By citing Paul in regards to end of era scenario,
you (and many) denigrate God's Revelation.
Though he deemed himself Godly,
before Paul was converted, Paul was a servant of Satan.
Though they may think themselves Godly, all who teach others to heed Paul
in regard to end time scenario, ... thus downgrading the Revelation, are serving Satan.
Paul became a great servant of Jesus AFTER Jesus converted him,
but Paul knew NOTHING about our 2000 year later end time scenario.
Neither Paul nor those who looked up to Paul had any reason to know or worry about our 2000 year later end of era scenario. They cared about and needed to know about their own end time scenario. Not about an end time coming 2000 years later.
Before the Revelation was given to Him by God, even Jesus did not know the contents of the Revelation. When Jesus spoke the words recorded in Matt 24, Jesus did not know nor care about the our 2000 year later end time scenario which is revealed ONLY in the God-sent Revelation.