The book of second Thessalonians was likely written a few weeks after Paul's first letter because it seems he is addressing the same issues or questions the believers had asked about, and he looking to cover whatever he missed in his first letter.
Paul again commends the believers for holding on despite the persecutions they were going through. He also wanted to give them better clarity on the likely time of Jesus' second coming and they should we be doing in the meantime. In this second letter, Paul adds 2 key events that will take place before Jesus returns;
"Now, dear brothers and sisters, let us clarify some things about the coming of our Lord Jesus Christ and how we will be gathered to meet him. Don’t be so easily shaken or alarmed by those who say that the day of the Lord has already begun. Don’t believe them, even if they claim to have had a spiritual vision, a revelation, or a letter supposedly from us. Don’t be fooled by what they say. For that day will not come until there is a great rebellion against God and the man of lawlessness is revealed—the one who brings destruction."
2 Thessalonians 2:1-3 NLT.
He begins by saying he needed to clarify which meant they had misunderstood something in his first letter. The 2 events are; there will be a massive rebellion against everything God. Secondly, the antichrist, the man of lawlessness would reveal himself before Jesus returns. When the time for Christ's return begins to draw near, society as a whole will turn against God, every truth with foundation on the word of God will be turned upside down and rejected; the family, morality, right and wrong, the church, etc. Christians will be called names; racist, fascist and narcissist. Right will become wrong and wrong right. Do you perceive we are getting there gradually? It will become wrong to love and right to hate. Society will fight and protect the cause of those who commit sin and abuse those who call stand for the truth.
When things like these become the norm, you know the day is drawing near. When the antichrist finally reveal himself in person, Paul says;
"He will exalt himself and defy everything that people call god and every object of worship. He will even sit in the temple of God, claiming that he himself is God."
2 Thessalonians 2:4 NLT.
When a figure appears in the future, a man of intrigue with exceptional talent and results, someone who even works miracles. He installs himself to be worshipped as god so that anyone who refuses is hounded and killed, you know the day of Jesus Christ's return is drawing near.
However, he cannot also be revealed until a major event takes place;
[6] And you know what is holding him back, for he can be revealed only when his time comes. [7] For this lawlessness is already at work secretly, and it will remain secret until the one who is holding it back steps out of the way. [8] Then the man of lawlessness will be revealed, but the Lord Jesus will slay him with the breath of his mouth and destroy him by the splendor of his coming.
.2 Thessalonians 2:6-8 NLT.
As believers, we can hold on to Paul's advise;
[15] With all these things in mind, dear brothers and sisters, stand firm and keep a strong grip on the teaching we passed on to you both in person and by letter.
2 Thessalonians 2:15 NLT.