2 Thessalonians 2:4 says that the man of sin/lawlessness will take his seat in God’s temple and declare himself God. So what does it mean that he will sit in “the temple of God?” Baptistic Dispensationalists take this to be a physical temple in Jerusalem. For one example, John MacArthur says in the future the man of sin will go into the physical [rebuilt] Jerusalem temple half-way through the seven-year period of tribulation, which will inaugurate the last half of the tribulation called the Great Tribulation.
I believe a Reformed interpretation on this phrase (the temple of God) is more biblical than a dispensational one. For example, Anthony Hoekema explains it like this:
“The expression ‘take his seat in the temple of God’ should not be understood as implying that there will once again be a literal Jewish temple at the time of Christ’s return…. The expression is probably best understood as an apocalyptic description of the usurpation of the honor and worship which is properly rendered only to God.” (Anthony Hoekema, The Bible and the Future, p. 160).
Herman Ridderbos explained it similarly:
“With the temple certainly the temple at Jerusalem is in the first instance to be thought of. One must not, however, fail to appreciate the apocalyptic character of the delineation. That which is still hidden, which as future event is still incapable of description, is denoted with the help of available notions borrowed from the present. To sit in the temple is a divine attribute, the arrogating to oneself of divine honor. No conclusions are to be drawn from that for the time and place in which the man of sin will make his appearance.” (Paul: An Outline of His Theology, p. 520-21)
“Old Testament references to God’s heavenly temple are also found in Psalms 11:4, 18:6, 103:19, and Habakkuk 2:20. If the reference is to God’s heavenly abode, ‘to sit’ is a metaphorical way of saying that the lawless man exalts himself to the place of a god. In the same way that the king of Babylon aspired to set his throne in heaven (Is. 14:13-14), and the king of Tyre proclaimed, ‘I am God, I sit in the seat of the gods’ (Ez. 28:2, Acts 12:21-23), so this lawless ruler will boast that he has dispossessed God and has taken his place.” (The Theology of the Old Testament, p. 575).
Finally, here’s F.F. Bruce:
“It may be best to conclude that the Jerusalem sanctuary is meant here by Paul and his companions, but meant in a metaphorical sense…. Few would have thought it necessary to think of a literal throne; it would have been regarded as a graphic way of saying that he [the man of sin] plans to usurp the authority of God.” (F.F. Bruce, 1 & 2 Thessalonians, p. 169).
Reformed theology sees the OT temple prophecies as being fulfilled in Christ, his people, and the new heavens/earth, not in a literal Jerusalem end-times temple. Therefore, the man of sin taking a seat in the temple of God is another way to say that this antichrist figure will proclaim himself to be God and seek to receive honor and worship as God.
But of course, God’s people need not fear, since Jesus will one day win the decisive and complete victory over the man of sin (2 Thes. 2:8).