So who is the one who holds it back? This isn't immediately obvious. Whenever this Scripture is discussed the answer always seems to be or is answered as the Holy Spirit. Then, some remarks are said to justify why. The Holy Spirit like the Father and the Son is All-Knowing, All-Powerful and with an Unlimited Nature. He will be here to His Full Being throughout the Tribulation. How does He relent? Or how does Jesus relent if it's Him instead?
Once that's been left with a big question mark, the church is usually the next answer. Once the church has been raptured that power of God to do good and maintain a hold on Satan will be gone. This does have some merit about it although angels who also do good and can be counted in the same way will still be present. However, God didn't need that many people in the past to do good so why does He need many now? If we look through the Scriptures we see examples like Gideon, Jephthah, Samuel, Moses & Elijah as examples. Actually, my thinking is that the church might be a way by which God sets up some of the relenting for a time. In this very Scripture passage above (verses 2-3) it says: "Whether by a prophecy or by word of mouth or by letter—asserting that the Day of the LORD has already come. Don’t let anyone deceive you in any way, for that day will not come until the rebellion occurs and the man of lawlessness is revealed." Also, the phrase within this 'until the rebellion occurs' is better described in other versions of the Bible. The Amplified Bible puts it like this: "Not to allow your minds to be quickly unsettled or disturbed or kept excited or alarmed, whether it be by some [pretended] Revelation of [the] Spirit or by word or by letter [alleged to be] from us, to the effect that the Day of the LORD has [already] arrived and is here. Let no one deceive or beguile you in any way, for that Day will not come except the apostasy comes first [unless the predicted great falling away of those who have professed to be Christians has come], and the man of lawlessness (sin) is revealed, who is the son of doom (of perdition)." So the falling away of Christians and revealing of man of perdition comes before the Day of the LORD. Also, we find in this passage (verses 1-12) in verse 8: "And then the lawless one will be revealed, whom the LORD Jesus will overthrow with the breath of His mouth and destroy by the splendour of His coming." The Lawless one will be revealed before the Second Coming of our LORD.
This gives a thought as to the correct order and how to think. The one who holds back will stop. There will be a great falling away of Christians. The man of perdition will be revealed. After these the Day of the LORD will come. Then, lastly, the Second Coming. There's small gaps and large gaps between all of these events as time moves forward. Does the Day of the LORD occur closer to the Second Coming or now? My (educated) guess is now.
At this time, in other words soon, the Two Witnesses (Moses & Elijah) will appear on the scene in Jerusalem. (Revelation 11:1-12.) (The Two Witnesses.) "I was given a reed like a measuring rod and was told, “Go and measure the Temple of God and the altar, with its worshipers. But exclude the outer court; do not measure it, because it has been given to the Gentiles. They will trample on the Holy City for 42 months. And I will appoint My Two Witnesses, and they will prophesy for 1,260 days, clothed in sackcloth.” They are “the two olive trees” and the two lampstands, and “they stand before the LORD of the earth.” If anyone tries to harm them, fire comes from their mouths and devours their enemies. This is how anyone who wants to harm them must die. They have power to shut up the heavens so that it will not rain during the time they are prophesying; and they have power to turn the waters into blood and to strike the earth with every kind of plague as often as they want. Now when they have finished their testimony, the beast that comes up from the Abyss will attack them, and overpower and kill them. Their bodies will lie in the public square of the Great City—which is figuratively called Sodom and Egypt—where also their LORD was crucified. For three and a half days some from every people, tribe, language and nation will gaze on their bodies and refuse them burial. The inhabitants of the earth will gloat over them and will celebrate by sending each other gifts, because these two prophets had tormented those who live on the earth.
But after the three and a half days the breath of life from God entered them, and they stood on their feet, and terror struck those who saw them. Then they heard a loud voice from Heaven saying to them, “Come up Here.” And they went up to Heaven in a cloud, while their enemies looked on." This describes their ministry. The event mentioned at the end of this passage is probably the rapture. At the beginning of the passage, however, we see the Lawless one's kingdom set up. At this time, these Two start their ministry. They bring down fire on their enemies. They will shut up the heavens. It won't rain. Will turn waters to blood. Strike earth with plagues. This is TWO people. Also, they are definitely not going or relenting! They are coming as stated and appointed in Revelation passage. Pursue and afflict as it says in Psalm 83 also a Psalm describing these Two, the hidden ones (verse3).
So who is it that relents? MIKaAL (or MICHAEL). He, the archangel, is the one who relents. We see this in the 2 Thessalonians passage already mentioned. His name means AL is Might-Subduer. That's what he does on behalf of believing Jews (or Israelites) and believing Gentiles (all called Christians in this age). Here are a few Scripture passages revealing his role.
(Daniel 12:1.)" `And at that time stand up doth Michael, the great head, who is standing up for the sons of thy people, and there hath been a time of distress, such as hath not been since there hath been a nation till that time, and at that time do thy people escape, every one who is found written in the book."
(Revelation 12:7-9.) "And there came war in the Heaven; Michael and his messengers did war against the dragon, and the dragon did war, and his messengers, and they did not prevail, nor was their place found any more in the heaven; and the great dragon was cast forth--the old serpent, who is called `Devil,' and `the Adversary,' who is leading astray the whole world--he was cast forth to the earth, and his messengers were cast forth with him."
Jude also mentions him. (Verse 9.) "But even the archangel Michael, when he was disputing with the devil about the body of Moses, did not himself dare to condemn him for slander but said, “The LORD rebuke you!”
So we see the role of MIKaAL. This is what happens after he relents. (Revelation 12:13-14.) "When the dragon saw that he had been hurled to the earth, he pursued the woman who had given birth to the male child. The woman was given the two wings of a great eagle, so that she might fly to the place prepared for her in the wilderness, where she would be taken care of for a time, times and half a time, out of the serpent’s reach." The woman is spiritual Israelites (Christians in this age) who aren't in the place of safety. As stated, the devil will pursue or persecute these - the foolish virgins of Matthew 25 and the great multitude which no one could count of Revelation 7:9-10. The woman, however, who gave birth to the man child (the 144,000) escapes to Selah Petra to be protected for her time by the LORD.
(Revelation 2:24-29.) "Now I say to the rest of you in Thyatira, to you who do not hold to her teaching and have not learned Satan’s so-called deep secrets, ‘I will not impose any other burden on you, except to hold on to what you have until I come.’ To the one who is victorious and does My Will to the end, I will give authority over the nations— that one ‘will rule them with an iron sceptre and will dash them to pieces like pottery’—just as I have received authority from My Father. I will also give that one the morning star. Whoever has ears, let them hear what the Spirit says to the churches." Amen.