“Let no one deceive you in any way. For that day will not come, unless the rebellion comes first, and the Man of Lawlessness is revealed, the son of destruction. . . .For the mystery of lawlessness is already at work.” (2 Thessalonians 2:3, 7)
Who was the Man of Lawlessness/Man of Sin of 2 Thessalonians 2:3-8? Let’s first note the context:
1. The “mystery of lawlessness” was already at work when Paul was writing this letter (2 Thessalonians 2:6-7).
2. The Man of Lawlessness would take his seat in the temple (2 Thessalonians 2:4).
3. Paul promised his readers that God would soon grant THEM (his first century readers) relief from persecution (2 Thessalonians 1:4, 7)
If Paul was a true prophet, these things happened while at least some of his readers were still alive. So, we should be looking for a first-century culprit―before the destruction of the temple in AD 70. To place this prophecy’s fulfillment thousands of years later does violence to the text.
Most Christians fail to appreciate the upheaval among the Jews inside the walls of Jerusalem prior to and during the Roman siege in AD 66-70. A group called the Zealots, a Jewish rebel terrorist group, perpetrated a civil war to incite the Jewish leadership to fight the Romans. Another rebel faction, the Idumeans (i.e., Edomites, an outside group with some Jewish heritage) aided the Zealots for a time. If it weren’t for these murderous rebels, the Jews might well have maintained peace with Rome.
Jerusalem was in a period of absolute mayhem in the AD 60’s, culminating in the Jewish-Roman War of AD 66-73. The Zealot rebels were against the Romans as well as against moderate factions of Jewish leadership. The rebels themselves were not united. It seemed that everybody was fighting each other.
Ananus ben Ananus (“ben” means “son of”), leader of the moderate faction and the high priest in 63 AD, wanted peace. (Ananus is also spelled Ananias.) But the Zealots were increasingly belligerent by AD 66, and incited the Romans to war. While war was waging, in AD 68, the Idumeans assassinated Ananus and another former Jewish high priest (Joshua ben Gamala). (Of note, Ananus was the leader who ordered the execution of James, the brother of Jesus, in AD 62.) Thousands more were murdered by the various rebel forces.
There were seeds of the rebellion brewing even while Paul was writing to the Thessalonians in the early-to-mid 50s AD. Hezekiah the Zealot was already stirring the pot in AD 47.
The Man of Lawlessness could be any of several fellows, including four Zealot leaders:
Eleazar ben Ananus was the ruthless original instigator of the rebellion. He was the captain of the temple guard, second in command to the High Priest. His lawless deeds make him a strong candidate for the Man of Lawlessness. He literally sat in the temple and acted lawlessly in the manner the Apostle Paul describes in 2 Thessalonians 2. Josephus in Wars of the Jews  railed against Eleazar as a tyrant, but more than that―acting as if he were a god in his lawlessness, abominations, and atrocities.
Eleazar apparently was responsible for convincing the priests to discontinue the practice of accepting offerings on behalf of aliens, namely the Roman rulers. This challenged the Roman authorities and signaled the revolt against Rome. In AD 66 Eleazar’s soldiers murdered Roman soldiers in the city who had laid down their arms.
The Roman army, under Cestius Gallus, then attacked Jerusalem to put down the rebellion, but retreated. The Jews claimed victory. But the victory was short-lived as Nero sent his army under Vespasian and his son Titus to siege Jerusalem in AD 67, finally destroying the city and temple in AD 70. Eleazar also led the Idumeans for a time. He was the son of Ananus, the most powerful former high priest at the time, and brother of Caiaphas.
John Levi of Gischala is another strong possibility. As one of the Zealot leaders, he set himself up in the temple as the Jewish Savior (as God). He desecrated the temple in about AD 68, including emptying vessels of sacred wine and oil used in temple rituals and melted down many sacred utensils, etc. He also plundered the people, even burning their storehouses of food, causing a great famine that starved tens of thousands to death.
Eleazar ben Simon (another radical who was unable to establish unity with John of Gischala). Eleazar made the temple his central command post for the entire first half of the Jewish-Roman War, and oversaw many lawless acts in the temple.
Simon ben Gioras was leader of yet another murderous Jewish rebel group that initially opposed the Zealots, but as the Roman attack became more serious, he aligned with the Zealots against the Romans. Each of the previous three commanded their own separate regiment of soldiers.
Others argue that Nero was The Man of Lawlessness. Nero was the Roman emperor from AD 54-68. The proponents of Nero argue that the description of the man of lawlessness echoes that of Daniel’s “little horn” (Daniel 7:8, 20, 21; 9:9-12; cf. 11:31, 36) and foreshadows John’s description of the beast from the sea (Revelation 13:1-8)―characters which can be identified as Nero.
Titus, the Roman general that conquered Jerusalem, is also a possibility. Like Antiochus Epiphanies 200 years before him, Titus defiled the temple in AD 70 by offering pagan sacrifices and setting himself up as “imperator” (supreme ruler)―before his men destroyed the temple. (Josephus’ Wars of the Jews 6.6.1).
The rebels even burned the stores of food, attempting to starve their moderate Jewish opponents into submission to fight the Romans, creating great hardship for the Jews. This civil war was ultimately devastating to the Jews and their stand against the Romans.
But wait! Wasn’t this supposed to happen at Jesus’ Parousia―at the Day of the Lord (2 Thessalonians 2:1-2, 8)? Well, notice in 2 Thessalonians 2:2 that many Christians thought that the Day of the Lord HAD ALREADY HAPPENED. They obviously had a different idea about what the Day of the Lord was than many modern Christians. They did not think it meant the end of the world.
The Day of the Lord is used throughout the Old Testament about God’s judgments on various nations (Isaiah 13:9; Jeremiah 46:10; Lamentations 2:22; Ezekiel 30:2-4; Amos 5:18-22; etc.). Paul’s imminent first-century Day of the Lord was the coming judgment against apostate Old Covenant Israel for her sins and failure to accept Jesus as Messiah (Matthew 23:29-24:2). This happened in AD 70 when Jerusalem was destroyed and the temple reduced to rubble. The gospels make it clear that the Day of the Lord would come upon the Jews (Matthew 3:11-12; 8:12; 10:23; 16:27-28; 21:33-46; 22:7; 23:34-24:2; Luke 19:41-44; 21:20-24 etc.)
Did Jesus really “come” in AD 70? Well, He certainly promised that his Parousia (effective divine presence) would be while some in the first century were still alive (Matthew 10:23; 16:27-28; 24:29-34; 26:64; John 21:22; Revelation 1:1-3; 22:6-20). If you can accept that Jesus came IN JUDGMENT in AD 70, then all of the passages above fit perfectly. You don’t have to make excuses for Jesus, Paul, or the other writers of the New Testament. The relief from persecution by the Jews promised by Paul to the Thessalonian Christians in the first century happened when the Roman army decimated the Old Covenant order and the Jewish ability to persecute Christians. This is precisely how God extracted his judgment in the Old Testament―sending an opposing army to conquer the offenders.
See these helpful links for more information about these things:
 You can find information about all of the characters listed in this article on the world wide web. (Note: If you do research on these figures, you will find it confusing because the spelling is not consistent. For example, Ananus is also spelled Annas or Ananias.)
 Wars of the Jews 2:455 (2.17.10); 4:162-163 (4.3.10); 4:201 (4.3.12); 4.323 (4.5.2); 4.388 (4.6.3); 5:14-19 (5.1.3); 5:402 (5.9.4); 6:110 (6.2.1); 6:126 (6.2.4).