Who or what is the Restrainer?

2 Thessalonians 2:1-8 ESV
(1)  Now concerning the coming of our Lord Jesus Christ and our being gathered together to him, we ask you, brothers,
(2)  not to be quickly shaken in mind or alarmed, either by a spirit or a spoken word, or a letter seeming to be from us, to the effect that the day of the Lord has come.
(3)  Let no one deceive you in any way. For that day will not come, unless the rebellion comes first, and the man of lawlessness is revealed, the son of destruction,
(4)  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.
(5)  Do you not remember that when I was still with you I told you these things?
(6)  And you know what is restraining him now so that he may be revealed in his time.
(7)  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.
(8)  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.
From the Believer’s Bible Commentary:

2:6   They knew what was restraining the full and open manifestation of the man of sin, and what would continue to restrain him until the appointed time.
This brings us to the third great unanswered question in this chapter. The first is, “What is the apostasy?” The second is, “Who is the man of sin?” The third is, “What or who is the restrainer?”
In the first part of verse 6, the restrainer is described in an impersonal way ... what is restraining. But then in verse 7 it is a person—He who now restrains. E. W. Rogers puts it clearly:

It is Something and Someone who wittingly, purposefully, and designedly holds it in check with the view to ensuring that the Man of Lawlessness is revealed in his own proper time.

Seven of the more common views as to the identity of the restrainer are: (1) the Roman Empire, (2) the Jewish State, (3) Satan, (4) the principle of law and order as found in human government, (5) God, (6) the Holy Spirit, and (7) the true church as indwelt by the Spirit.
The Holy Spirit indwelling the church and the individual believer seems to fit the description of the restrainer more completely and accurately than any of the others. Just as the restrainer is spoken of as Something and Someone in this chapter, so the Spirit is spoken of in John 14:26, John 15:26, John 16:8, John 16:13-14 as both neuter (the Holy Spirit) and masculine (He). As early as Genesis 6:3, the Holy Spirit is spoken of in connection with the restraint of evil. Then later He is seen in this same role in Isaiah 59:19 b, John 16:7-11, and 1 John 4:4.
It is by the indwelling Spirit that believers are the salt of the earth (Matthew 5:13) and the light of the world (Matthew 5:14). Salt is a preservative, but it also hinders the spread of corruption. Light dispels darkness, the sphere in which men love to perform their evil deeds (John 3:19). When the Holy Spirit leaves the world as the permanent Indweller of the church (1 Corinthians 3:16) and of individual believers (1 Corinthians_6:19), the restraint of lawlessness will be gone.

I would agree with the above assessment and add this to it.  Jesus speaks of the Church as not being subordinate to any evil:
Matthew 16:18 ESV
(18)  And I tell you, you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it.

Yet in this passage:
Revelation 13:5-7 ESV
(5)  And the beast was given a mouth uttering haughty and blasphemous words, and it was allowed to exercise authority for forty-two months.
(6)  It opened its mouth to utter blasphemies against God, blaspheming his name and his dwelling, that is, those who dwell in heaven.
(7)  Also it was allowed to make war on the saints and to conquer them. And authority was given it over every tribe and people and language and nation,
it seems that the Beast has conquered the saints. The only way to make sense of this is to understand that the Church has been removed prior to the appearance of the Antichrist and the “saints” spoken of here in Revelation are Tribulation saints i.e. those saved after the rapture.
Thus, once again, the doctrine of the rapture takes into account and fully explains these passages.