The restrainer
Who or what restrains the lawless one, and what the gathering and apostasy are.
2 Thessalonians 2:1-12
1Now, brothers, concerning the coming of our Lord Jesus Christ and our gathering together to him, we ask you
2not to be quickly shaken in your mind, and not be troubled, either by spirit, or by word, or by letter as if from us, saying that the day of Christ has already come.
3Let no one deceive you in any way. For it will not be, unless the rebellion comes first, and the man of sin is revealed, the son of destruction,
4he 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.
5Don’t you remember that, when I was still with you, I told you these things?
6Now you know what is restraining him, to the end that he may be revealed in his own season.
7For the mystery of lawlessness already works. Only there is one who restrains now, until he is taken out of the way.
8Then the lawless one will be revealed, whom the Lord will kill with the breath of his mouth, and destroy by the manifestation of his coming;
9even he whose coming is according to the working of Satan with all power and signs and lying wonders,
10and with all deception of wickedness for those who are being lost, because they didn’t receive the love of the truth, that they might be saved.
11Because of this, God sends them a working of error, that they should believe a lie;
12that they all might be judged who didn’t believe the truth, but had pleasure in unrighteousness.
What each tradition says
Rabbinic / Midrashic
Louis Ginzberg argues it is unreasonable to take “he who now restraineth” as designating the Roman government or the emperor himself. The iniquitous one, the incarnation of evil upon earth, is not a political personage held back by Roman power.
Louis Ginzberg, Antichrist (The Jewish Encyclopedia, 1901-1906) — Antichrist in Pauline Writings (on 'he who now restraineth')
Patristic
Augustine says it is not absurd to believe that the apostle’s words, “Only he who now holds, let him hold until he is taken out of the way,” refer to the Roman empire. On this reading, it is as if it were said, “Only he who now reigns, let him reign until he is taken out of the way.”
Augustine, The City of God, Book XX (NPNF ser. 1, vol. 2, Dods trans.) — Book XX, ch. 19 (on 2 Thessalonians 2 and the hinderer)
Catholic
Majority viewAnthony Maas states that the impediment is the Roman Empire. He says the main event being impeded is the "man of sin." This reading is associated with most Latin Fathers and later interpreters.
Anthony Maas, The Catholic Encyclopedia, vol. 1 (New York: Robert Appleton Company, 1907), s.v. 'Antichrist' — Antichrist -- In the Pauline epistles (the katechon)
Orthodox
Chrysostom asks what withholds, or hinders, him from being revealed. He notes that some say it is the grace of the Spirit, while others say it is the Roman empire. He says he most of all accedes to the Roman empire.
John Chrysostom, Homilies on Second Thessalonians (NPNF ser. 1, vol. 13) — Homily IV on 2 Thess. ii. 6-9 (the restrainer)
Reformed / Protestant
Calvin’s opinion is that Paul’s intention was different from this: the doctrine of the gospel would need to be spread hither and thither, until nearly the whole world were convicted of obstinacy and deliberate malice. This is what Calvin takes Paul to mean here.
John Calvin, Commentary on Philippians, Colossians, and Thessalonians (Calvin Translation Society) — Comment on 2 Thessalonians 2:6 (the impediment)
Evangelical
Wesley identifies "that which restraineth" as the power of the Roman emperors. He says that when this restraining power is taken away, the wicked one will be revealed, at his appointed season and not before.
John Wesley, Wesley's Notes on the Bible, 2 Thessalonians (Wesley Center Online) — Notes on 2 Thessalonians 2:6-7