Paul urges the Thessalonians not to be panicked by the thought that "the day of the Lord" that they've all been waiting for has already come and gone. Um, we don't think it's likely that they would miss all the angels and trumpets and such, Paul.
He says that they shouldn't let anyone trick them about this. Especially not "the man of sin." Ominous, much?
This guy puts himself above God. In fact, he tells everyone he is God.
Paul says he already told them all about this dude and that something is holding this sinful guy back right now. The Thessalonians know what, but Paul isn't clueing us in.
Pretty soon, the Man of Sin will be shown for what he is, and Jesus will come down and kill him "with the breath of his mouth."
Maybe try brushing before you descend from Heaven next time, Jesus.
Basically, this is the work of Satan. He's in charge of all things bad and deceitful, and he loves it when people refuse to follow Jesus. It's his favorite.
But it's God who's making these non-believers buy into these lies. (Wait, seriously, God?) Later, he will condemn them for it.
Lucky for the Thessalonians, God has destined them for all kinds of good things. God spoke to them through the words of his messengers (i.e., Paul) and they accepted it.
Now, if they just hold onto the things that they've been taught, it'll be smooth sailing from here on out.