The context of this passage is to comfort the Thessalonians who were under the false impression that they were going through the tribulation period. Paul writes to correct their theology and sets out a few guidelines of the tribulation period. Within this explaination is a prophecy regarding a deception that would come upon the whole world.

God desires that all be saved and come to the knowledge of the truth (1 Tim. 2:4-6). But when people refuse to entertain the truth, He lets them pursue and experience the consequences of falsehood (cf. Rom. 1:18-25). In fact God, as the Judge of men, begins this judgment at the moment of their rebellion and subjects them to the powerful delusion (energeian planes) which comes from choosing error over truth. They choose to believe the lie and God sends them the delusion that is inherent in their choice. This powerful judgment from God is justified by the unbeliever's decision to refuse the truth. "The lie" is the claim that the man of lawlessness is God. (The Bible Knowledge Commentary, p. 720)