Paul gets to the core matter of the 2nd letter in chapter 2. Apparently someone has been teaching that the "Day of the Lord" has already come and that the resurrection has already happened (see verse 2). It appears that an alleged prophecy or a false letter from Paul has been presented to the church arguing that the second coming of Jesus has already happened. In the first letter Paul had to argue that the 2nd coming of Jesus was imminent (could happen at any time). In the second letter Paul has to argue against the false teaching that it has already happened. He argues that there are events (the whole discussion of the "man of lawlessness") that must take place before Jesus returns. About all we Christians can say on the subject of Jesus 2nd coming from the Thessalonians correspondence is that Jesus is coming back so be ready; any more would be pure speculation.
I find a lot of people trying to jettison the traditions of the Christian faith to make the faith more palatable and more interesting to the 21st century world. There is a fine distinction that needs to be made. In verse 15 Paul writes "stand firm and hold fast to the traditions that you were taught by us, either by word of mouth or by our letter". He is talking about the essential basic core understandings of the Christian faith. Someone once said that the true battle is in the Faith is the battle between the Tradition and traditional. Tradition (note the capital 'T') is the living faith of the dead -- the time honored faith and understanding that has been handed down to us from apostolic times. Traditional (small 't') is the dead faith of the living. Traditional is hanging on to particular ways of doing things, certain modes of worship, certain styles of music, certain rituals and methods because "we've always done it this way." For the Christian, who is called to present the gospel to every nation, language, people (and I would add in every time and context) presenting the authentic Tradition of the Christian faith in ways that are contextually and culturally relevant is always the challenge.Many traditional ways of doing things (robes, printed bulletins, committee structures, institutional structures, order of worship . . . you get the idea) may or may not help with with this goal.
What is at the core of your faith? Why? In my journey in Christ (coming up on 40 years) I have found that keeping the core (essentials) in the core and the peripherals (non essentials) on the periphery is a constant challenge. The tricky part is recognizing what is in the core and what is on the periphery.