1 Thess. 4:13-18 Paul reminds them that the risen Lord Jesus will come again. When he does, those believers who have died will be given a renewed life on earth. And those believers who are living at that time will go up into the clouds to welcome the Lord Jesus coming down to earth from heaven.
The ‘Day of the Lord’ will come unexpectedly (1 Thess. 5:1)
1 Thess. 5:1-11 Paul says this ‘Day of the Lord’ will come unexpectedly like a thief in the night. So believers should be expectant, living a sober and righteous life. They should ‘wear’ faith and love like a soldier’s breastplate and ‘put on’ the hope of salvation like a helmet (see also Ephesians 6:13-17, where Paul expands on this picture of the ‘armour of God’).