The second condition that sucks the power out of a church or out of the life of a Christian is deception.   Deception is when you choose to believe a lie.  As bad as the 1st debilitating condition was; discouragement, deception has done far more to destroy the church than discouragement has. 
In this passage of Scripture, Paul warns the Thessalonian believers that a lie is circulating in their church.  Then he tells them how to deal with it.