Rulers who took divine titles and entered sanctuaries
Hellenistic and Roman167 BC for Antiochus IV; AD 40 for the order of CaligulaThe parallel
Antiochus IV struck coins calling himself God Manifest and desecrated the Jerusalem sanctuary in 167 BC, the event Daniel's abomination language stands behind. In AD 40 the emperor Caligula ordered his own statue set up in the same sanctuary. Josephus tells the story at length and Philo, who led an embassy to the emperor, wrote his own account. The order was delayed by the governor of Syria and lapsed when Caligula was killed in AD 41. Paul is writing about ten years after that crisis to people who had heard of it.
Read honestly
A real and close background for 2:4, and it fixes the register of the verse. A ruler installing himself as God in the sanctuary was a thing that had happened once and had almost happened again inside living memory, so Paul's readers took the sentence as a description of an event. Paul writes of a figure still to be revealed (2:3).

