"Who opposes and exalts himself above all that is called God, or that is worshipped; so that he sits in the temple of God, showing himself that he is God." II Thess 2:4 (The Greek text here doesn't demand that he actually claim to 'be' God, but rather that he is showing off by taking the honors 'due' God.)
The temple was a physical building for the Jews pre-Christ. But what is the temple of God now? The church. (I Cor 3:16-17, Heb 8:4-5, Heb 9:24, Acts 7:28, I Pet 2:5, etc.)
"And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people.'" II Cor 6:16
So we are not necessarily looking for someone to somehow sit in a Jewish re-built temple and declare himself God or higher than God. That would actually seem less sensible for the Christians who would read Revelation - interpreting prophecy through the limited lens man had pre-Christ or artificially conflating this prophecy with others given to the Jews long ago.
We could also be looking for is someone who sets himself up within the church above every god or object of worship, wishing to be the center of the church and treated as 'God.' We could even be looking for someone outside the church who seeks to usurp God in the minds and hearts of believers.
Elicott's commentary sums up how broad this prophecy could be like this:
"Though the image is drawn from the Jewish temple, we may say with some confidence that St. Paul did not expect the Antichrist as a prose fact to take his seat in that edifice. Neither is the metaphor to be pressed into a mere synonym of “the Church” (
1 Corinthians 3:17). The words, so far need not necessarily mean that the Man of Sin will make special claims upon the Christian community as such. Rather, the whole phrase, “taking his seat in the temple of God,” is a poetical or prophetical description of usurping divine prerogatives generally: not the prerogatives of the true God alone, but whatever prerogatives have been offered to anything “called God.” Though the prophecy might be fulfilled without any symbolical act (e.g., of assuming any material throne), yet the spontaneousness (“himself ) and the openness (“showing himself”) seem so essentially parts of the prophecy as of necessity to imply that the Man of Sin will make formal claim to occupy that central seat in men’s minds and aspirations which is acknowledged to be due to God alone."
https://biblehub.com/commentaries/2_thessalonians/2-4.htm
So
the Papacy is neither a guaranteed nor impossible candidate here. The Papacy sets itself up as the center of the Universal church, with the Pope as the highest member - even called 'Holy Father.' The Pope sits on a throne and others often bow to him, prostrate before him, bow and kiss, etc.
"The leader of the Catholic church is defined by the faith as the Vicar of Jesus Christ (and is accepted as such by believers).
The Pope is considered the man on earth who represents the Son, who "takes the place" of the Second Person of the omnipotent God of the Trinity....Catholics believe this and therefore call him "Holy Father" or "Your Holiness." (John Paul II, Crossing the Threshold of Hope, p. 3, 1994)." goo.gl/MJcwDa
Bull of Pope Boniface VIII:
"Therefore, of the one and only Church there is one body and one head, not two heads like a monster; that is,
Christ and the Vicar of Christ, Peter and the successor of Peter,... Therefore whoever resists this power thus ordained by God, resists the ordinance of God [Rom 13:2], unless he invent like Manicheus two beginnings, which is false and judged by us heretical, since according to the testimony of Moses, it is not in the beginnings but in the beginning that God created heaven and earth [Gen 1:1]. Furthermore, we declare, we proclaim,
we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff."
http://www.americancatholictruthsociety.com/docs/unamsanctum.htm
"Christ entrusted his office to the chief Pontiff; (Mat 16:18, Mat 24:45) but
all power in heaven and in earth had been given to Christ; (Mat. 28:18) therefore the chief Pontiff, who is his vicar, will have this power."
(Extravagantes, Decretal. Greg. IX. de Transl. lib. i. tit. 7. c. 3.
'Quanto personam,' Pope Innocent III)
goo.gl/kz2feu
While various Bible scholars and commentators have come to different views and theories on 'who is the Antichrist' - the Papacy is a common theory (very common during the Reformation) and certainly not implausible. It is not without history and scripture to support the view, though there are other possibilities as well.