he who opposes and exalts himself against all that is called God or that is worshiped; so that he sits as God in the temple of God, setting himself up as God.
Sitteth in the temple of God ...  There can be no way that this is a reference to the Jewish temple.  Paul, who wrote the Corinthians that "Ye are the temple of God," would never have made that den of thieves and robbers in Jerusalem the "temple of God" historically.  First, it means the church of Jesus Christ; but in context it means the apostate church of Jesus Christ, a deduction that is mandatory from the fact of the apostasy being Paul's subject in this paragraph.  Therefore, whenever and wherever the "man of sin" appears it will be in the church apostate!