The Restrainer is a character who appears in 2 Thessalonians 2. Paul describes him as restraining the mystery of lawlessness and wrote that when the Restrainer is taken out of the way the lawless one will be revealed. 
And you know what is restraining him now so that he may be revealed in his time. For the mystery of lawlessness is already at work. Only he who now restrains it will do so until he is out of the way. And then the lawless one will be revealed, whom the Lord Jesus will kill with the breath of his mouth and bring to nothing by the appearance of his coming.
(2 Thess. 2:6–8)
Earlier in the chapter, Paul explained that the "man of lawlessness" takes his seat in God's temple and proclaims himself to be God.
Opinions vary as to how this passage ties in with other passages of eschatology, and the identity of the characters here.
Dispensational premillennialists claim that this is "the abomination that causes desolation" referred to in the prophecies of Daniel (see Daniel 9:27), and assert that the "lawless one" is the Antichrist.
The Protestant historicist tradition claims that the "lawless one" is the Pope, exalting himself in the Church, which is the temple of God. For instance, John Calvin, in his commentary on 2 Thessalonians, puts forward this idea.  Ian Paisley is an example of a modern Protestant who holds to this view. Calvin refers to the Restrainer via the interpretation of St. John Chrysostom, who suggsted either the Holy Spirit or the Roman Empire as the Restrainer, and following Chrysostom's conclusion but not his reasoning, leant towards the Roman Empire as the Restrainer.
Classical amillennialists agree with the identification of the temple with the Church, but instead would argue that the releasing of the man of lawlessness is connected with the great apostasy, which is to occur just before the return of Christ. The identity of the Restrainer is not obvious, but could be the Holy Spirit or the preaching of the gospel.^ ^