The phrase 'all that is called God' is difficult to analyze in English because the proper noun 'God' ('God' with a capital G) is known to be reserved to him who possess the divine identity and divine substance (cf. 1 Corinthians 8:6; Galatians 4:8).
In Trinitarian theology, this would be construed as the individual who self-identifies (claims) as someone above the Trinity. This shows ultimate power, a power beyond the Godhead which is nothing but a mere show though it may appear real due to its controlling influence ( cf: 2 Thessalonians 2:9; Revelation 13:3).
The phrase 'πάντα λεγόμενον θεὸν' may refer to the false gods (1 Corinthians 8:5). The exegetical evidence is the Greek word sebasma that is also used in Acts 17:23 in its plural form. The following English translations show this understanding:
(English Standard Version) who opposes and exalts himself against every
so-called god or object of worship, so that he takes his seat in the
temple of God, proclaiming himself to be God.
(Holman Christian Standard Bible) He opposes and exalts himself above
every so-called god or object of worship, so that he sits in God's
sanctuary, publicizing that he himself is God.
(NET Bible) He opposes and exalts himself above every so-called god or
object of worship, and as a result he takes his seat in God's temple,
displaying himself as God.
The Ethiopic version renders εἰς τὸν ναὸν τοῦ θεοῦ as "in the house of God" (recalling 1 Corinthians 3:16) and thus, the subject who deifies himself as the one true God is to be found within the church.
Gill's Exposition of the Entire Bible
so that he as God sitteth in the temple of God; not in the temple of
Jerusalem, which was to be destroyed and never to be rebuilt more, and
was destroyed before this man of sin was revealed; but in the church
of God, so called, 1 Corinthians 3:16 the Ethiopic version renders it,
"in the house of God"; for antichrist rose up out of, and in the midst
of the church; and it was a true church in which he first appeared,
and over which he usurped power and authority; though it has been so
corrupted by him, as now to be only nominally so; here he sits, and
has homage done him by his creatures, as if he was a god, and is not
only styled Christ's vicar, but a god on earth, and our Lord God the
Pope; so in the triumphal arch at the entry of Pope Sixtus IV, these
lines were put, "oraculo vocis, mundi moderaris habenas, et merito in
terris crederis esse Dens"; the sense is, that he governed the world
by his word, and was deservedly believed to be God on earth; and their
canon law (Gratian. Decret. dist. 96. can. "satis".) says,
"it is clearly enough shown, that the Pope cannot be loosed or bound
by any secular power; since it is evident that he is called God by
that pious prince, Constantine, and it is manifest that God cannot be
judged by men:''
and Pope John 22 is expressly called ( Extrarag. "cum inter".) "our
Lord God the Pope": the Ethiopic version reads, "he shall say to all,
I am the Lord God"; see Ezekiel 28:2, the Alexandrian copy, and some
others, and the Vulgate Latin version, leave out the phrase, "as God",
but the Syriac retains it: however, the same blasphemy is expressed in
the next clause,
This seems probable but one thing is for sure, like Satan, the man of lawlessness will claim Godhood but his destruction is sure (2 Corinthians 2:8).
12 How art thou fallen from heaven, O Lucifer, son of the morning! how
art thou cut down to the ground, which didst weaken the nations!
13 For thou hast said in thine heart, I will ascend into heaven, I
will exalt my throne above the stars of God: I will sit also upon the
mount of the congregation, in the sides of the north:
14 I will ascend above the heights of the clouds; I will be like the
Isaiah 14:12-14 (KJV)