So Jesus hadn’t yet returned, and the Thessalonians hadn’t been left behind. Instead, and while they endured some significant persecution, Paul encouraged them to be patient. Jesus would return with honour for them, and justice for the persecuted :
2 Thessalonians 1:7-10 And God will provide rest for you who are being persecuted and also for us when the Lord Jesus appears from heaven. He will come with his mighty angels, in flaming fire, bringing judgment on those who don’t know God and on those who refuse to obey the Good News of our Lord Jesus. They will be punished with eternal destruction, forever separated from the Lord and from his glorious power. When he comes on that day, he will receive glory from his holy people—praise from all who believe. And this includes you, for you believed what we told you about him.
That Bible blog 