Note 8 at 2 Th. 1:7: It seems to be odd instructions from Paul to tell those who are being persecuted to rest. The Greek word that was translated "rest" is ANESIS, which means "relaxation or figuratively relief" (Strong). It is this meaning of "relief" to which Paul is referring here.

It is a relief to know that all the injustices we see around us are someday going to be set right. The Lord will deal with every act of man and dispense true justice-no one will get by with anything. It might look that way from our human point of view, but the righteous Judge will have His day in court.

This is basically the reasoning that David used in Psalm 37. He said not to fret ourselves because of evildoers. There is coming a day when they will be cut down and the righteous will shine forth as the light. That's what Paul is referring to here.