Question: "Who / What is the restrainer in 2 Thessalonians 2:6?"
Answer: 2 Thessalonians 2:6-7 mentions a “restrainer” that is holding back sin and lawlessness. What or who that restraining force is, Paul does not say specifically in this letter. Many have speculated as to the identity of the restrainer, naming the restraining force as 1) human government; 2) gospel preaching; 3) the binding of Satan; 4) the providence of God; 5) the Jewish state; 6) the church; 7) the Holy Spirit; and 8) Michael. Whatever now restrains the Antichrist of vv. 3, 4, 8–10 from being revealed in the fullness of his apostasy and evil must be more than human or even angelic power. The power that holds back Satan from bringing the final apostasy and unveiling of his Satan-possessed false Christ (the Antichrist) must be supernatural. It must be God’s power in operation that holds back Satan, so that the “man of sin,” “the son of destruction” (v. 3) won’t be able to come until God permits it by removing the restraining power. The reason for the restraint was so that the Antichrist would be revealed at God’s appointed time and no sooner, just as was Christ (Gal. 4:4), because God controls Satan.
The Holy Spirit of God is the only Person with sufficient (supernatural) power to do this restraining. How does He do it? Through Christians, He works in society to hold back the swelling tide of lawless living. At some point, though, He will be removed from the earth, allowing sin to have dominion over mankind. This can only happen when the Church—all true believers in Jesus Christ—is removed. This is because the Holy Spirit lives within each believer. When the church leaves the earth at the rapture, the Holy Spirit will be taken out of the way in the sense that His unique lawlessness-restraining ministry—through God's people— will be removed (see Genesis 6:3).
The removal of the Restrainer at the time of the rapture must obviously precede the “day of the Lord.” But as soon as individuals again start placing their faith in Christ during the tribulation, the Holy Spirit will again be “present” in the world. His ministry of restraining, though, seems limited to the time before the tribulation.