exalt himself: Like other Roman cities, Thessalonica built temples to worship the emperor (king). This religion (cult) was the model for the worship described here.
he will seat himself in the temple of God: Antiochus Epiphanes polluted the temple in Jerusalem in 167 BC (see Daniel 9:27; 11:31; 12:11). This might have anticipated the event prophesied here (Matthew 24:15; Mark 13:14). It could also refer to Emperor Caligula's attempt to place his image in the temple in AD 40.
Alternatively, the temple might be a temple of the emperor. Here, of God could mean of the god. They will build this temple to honor the man of lawlessness. It is not necessarily the temple in Jerusalem.
proclaiming himself to be God (Ezekiel 28:2–10): In the first century AD, people often gave divine titles to the emperor.