A Traditionalist Roman Catholic (not one who would be an apologist for a general apostasy [the author is not a Sedevacantist]) wrote the following about 2 Thess 2:4:
“sits in the temple of God”:
αυτον εις τον ναον του θεου καθισαι, the verb καθισαι (“sits”) an aorist
infinitive to indicate one decisive and permanent act; while εις denotes
that he puts himself into the structure. The word ναον (“temple”) did not
usually refer to the temple proper but “only of the sacred sanctuary itself,
consisting of the Holy Place and the Holy of Holies; in classical Greek, used
of the sanctuary or cell of a temple where the image of the god was kept” (THR,
FRB). As such, this place is the inner sanctuary “of God” (του θεου),
far more exclusive than a pagan sanctuary of which there were many spread far
and wide. The question remains as to what particular “sanctuary” Paul has in
view. The “temple” could be the Jewish temple, which was still standing when
Paul wrote Second Thessalonians and thus may be prophetic of a future Jewish
temple when the man of sin appears. Paul could also be referring to the
sanctuary or inner sanctum of the visible Church, possibly the papal chair
itself. (Robert A. Sungenis, The Epistles to the Philippians,
Colossians, and Thessalonians [Catholic Apologetics Study Bible IX; State
Line, Pa.: Catholic Apologetics International Publishing, Inc., 2020], 94 n.
20, emphasis added)
In
his Understanding Paul, Richard Lloyd Anderson noted that:
Paul's central symbol of the
apostasy is the man of sin or lawlessness sitting "in the temple of God,
shewing himself that he is God" (2 Thes. 2:4). Pounds of pages have been
written about this being the Jerusalem temple, but that would be destroyed
within two decades and would have no one sitting in it. And what did that
temple mean to the Greek Gentiles or even to apostles in terms of their own
religion without Mosaic sacrifices? The real question is how Paul used the
word temple in his writing. Almost always he used it figuratively—occasionally
the body is a temple for God's Spirit, but usually the Church is the temple
of God. The members ("ye," older plural English for the plural
Greek) are "God's building" (1 Cor. 3:9), with Christ its foundation
(1 Cor. 3:11), or, in summary, "the temple of God" (1 Cor. 3:16).
Elsewhere Paul teaches about Christ as cornerstone, apostles as foundation, and
members fitting into their places as a "holy temple in the Lord"
(Eph. 2:21). And in one of his last letters, Paul still spoke of "the
house of God, which is the church of the living God" (1 Tim. 3:15). Paul
must define Paul, and his own words show that he was here referring to the Church.
. . .
Compare and contrast the following with the attempt to preclude
the "Temple/ναος = the Church" interpretation:
Although it is true that in 1 Cor.
3:16f. Paul employs ναός in a metaphorical sense to refer to the Christian as a
temple of God (cf. 1 Cor. 6:19), in the present context where no mention is
made of the believer and the indwelling of the Spirit of God as in 1
Corinthians, such an interpretation is highly unlikely. Jewish Christians as
well as Gentile Christians undoubtedly would have understood it as a reference
to the one true temple of God in Jerusalem, especially since the verse contains
an allusion to Dn. 11:31–36 and the desecration of the temple at Jerusalem by
Antiochus Epiphanes . . . (Charles A. Wanamaker, The Epistles to the
Thessalonians: A Commentary on the Greek Text [New International Greek
Testament Commentary; Grand Rapids, Mich.: Eerdmans, 1990], 246-47)