For the reason that Paul never states who the restrainer is. Therefore any discussion on who it may be must, by speculation and assumption, become a debate. Sorry.
A few questions then. God is not the author of confusion so why and how can something be said when there are no other scriptures to confirm it?
2 Corinthians 5:14-17
For the love of Christ constraineth us; because we thus judge, that if one died for all, then were all dead: [15] And that he died for all, that
they which live should not henceforth live unto themselves, but unto him which died for them, and rose again. [16] Wherefore henceforth know we no man after the flesh: yea, though we have known Christ after the flesh, yet now henceforth know we him no more. [17] Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
2 Corinthians 4:3-4 But if our gospel be hid, it is hid to them that are lost: [4] In whom the god of this world hath blinded the minds of them which believe not,
lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them.
Does the love of Christ then constrain the world? Because Jesus Christ warned if the world hated Me then it will hate you. In looking for some support of restraining in the word there was evil restrained from receiving good as the same as the natural man cannot receive the things of God for they are foolishness to him and neither can he know them. But consider Saul/Paul who testified of consenting to Stephens death (stoning) and wasting, persecution, imprisonment and destruction of the body of Christ. There was no restraining of evil until Paul could say “the love of Christ constrains”. They killed The prophets, they “did not know Him or they would not of killed Him” there was no restraining in “the love of Christ”. Saying the Antichrist cannot be revealed or have full reign until the Holy Spirit is removed ...The word says all were evil and wicked and gone out of the way, full(pregnant) with cursing. Ephesians 5:16 Redeeming the time, because
the days are evil.