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.
Read more:
http://www.gotquestions.org/restrainer.html#ixzz3DFohDUeS