The restrainer or who withholdeth. Who is the restrainer and what or who is removed or held? Using scripture, the key that unlocks this mystery. 

Selected scripture
2 Thessalonians 2:6
6 And now ye know what withholdeth that he might be revealed in his time. KJV

The Holy Spirit? The Holy Spirit is no mystery as He is talked  about and explained throughout scripture. He doesn’t need to be revealed.
If the Holy Spirit is that withheld, then, to withhold or restrain Him would mean to stop from living a life of God, to be able to learn from the Holy Ghost, all would be left with their own thoughts. Through other scriptures we see the Holy Spirit continues His work and is on going throughout all scripture.  
 
If that, that is restrained is the Church, the raptured, how would that effect those left behind. The Holy Spirit is a gift, we would have to ask why would our (their) gift be restrained? Or taken away.  If we are filled with the Holy Ghost. How would that work? 

Our root is Christ Jesus, and our life is hid in Him through the Holy Spirit, this is the secret of the Lord. The Christian life is as secret as life itself. To have it removed at anytime reality doesn’t make much sense. Not really. 

Who is doing the restraining? God?

The key to understanding.
At the time Satan is held, the antichrist will be revealed.
This person is given power and great authority by Satan.

Selective scripture
Revelation 13:2
and the dragon gave him his power, and his seat, and great authority. For 3 ½ years KJV

How is Satan withheld?

Revelation 12:7
7 And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, KJV

Satan is not seen in scripture again until the middle of the tribulation. This is how we know Satan is withheld for 3 ½ years. 

When is Satan withheld? At the time of the rapture.

Selective scripture
Revelation 12:4
4 and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born. KJV

Satan and his are restrained in heaven.

A something or a someone is to be revealed, Satan is gone and what takes his place is revealed, this is when all will know who is the first beast in Revelation 13.

Selected scripture, paraphrasing
2 Thessalonians 2:6
6 And now ye know what withholds = Michael, remember the war?
As to allow or bring into the picture, he that received Satan’s power and authority. 

that he might be revealed in his time. KJV