5 Surely you recall that I used to tell you these things while I was still with you.
Paul is reminding the church in Thessalonica of the teachings that he taught them while with them. He was reassuring them that they were not left behind because of the falling away (apostasy of false believers) and the Antichrist hasn’t arrived on the scene yet which precedes the Lord’s coming.
Side Note: I’m not saying that Christians will go through the Tribulation. I do not believe that according to the Scriptures. Once the great apostasy comes and the man of lawlessness appears into the world stage, Jesus raptures up the church before the start of the Great Tribulation.