you're not missing much. The bible contains two letters ("1 Thessalonians" and "2 Thessalonians") which were written by Paul and addressed to the Christian community in Thessalonica (in Greece). Paul was a Jewish religious figure who initially rejected the idea that Jesus was the Messiah, and persecuted anyone who did believe it. Probably just imprisonment, nothing as bad as replacing "Happy Christmas" messages with "Happy Holidays" like modern Christians are subjected to.

Anyway, he had some kind of mental breakdown on the road to Damascus, details are unclear, it was very hot and he was probably dehydrated so heatstroke is the likely cause. In any case, after that point he had a 180 and became a massive fan of Jesus and wrote a load of letters like this, and he was convinced that Jesus would be coming back very soon to enact violent retribution throughout the land. In Thessalonians 1, he says, "we who are alive, who are left until the coming of the Lord", which echoes what Jesus allegedly said, "Truly I tell you, this generation will not pass away before all these things take place."

Of course they were both wrong. The world did not end in their lifetimes, and that's why everyone today accepts that both of them were merely having a mental breakdown and, to be blunt, chatting shit. Right?