Someone had been spreading lies in Thessalonica to discredit this whole Jesus thing: some believers were worried that Jesus had come back, but they’d been left behind. Paul said No, Jesus will come back when the time is right, and it’s not yet:
1 Thessalonians 2:2-8 Don’t be so easily shaken or alarmed by those who say that the day of the Lord has already begun … For that day will not come until there is a great rebellion against God and the man of lawlessness is revealed—the one who brings destruction. He will exalt himself and defy everything that people call god and every object of worship. He will even sit in the temple of God, claiming that he himself is God … And you know what is holding him back, for he can be revealed only when his time comes. For this lawlessness … will remain secret until the one who is holding it back steps out of the way. Then the man of lawlessness will be revealed, but the Lord Jesus will kill him with the breath of his mouth and destroy him by the splendor of his coming.