The full text of 2 Thessalonians 2:6-8 (ASV) reads:
6 And now ye know that which restrains, to the end that he may be revealed in his own season. 7 For the mystery of lawlessness does already work: only there is one that restrains now, until he be taken out of the way. 8 And then shall be revealed the lawless one, whom the Lord shall slay with the breath of his mouth, and bring to nought by the manifestation of his coming…
What is being restrained in 2 Thessalonians 2 is lawlessness. The principles and teaching in Genesis 9 and Romans 13 are that restraining lawlessness (not sin) is the function of human governments. In 2 Thessalonians 2, Paul does not identify the restrainer as being the Holy Spirit because that is not the function of the Holy Spirit. Furthermore, he does not identify the restrainer as being the church either. Authoritarian governments, such as communist or Islamic governments, have been much more successful in restraining lawlessness than the church ever has been.
It is important to note that 2 Thessalonians 2:6-7 speaks of the restraining of lawlessness, not of evil or “the evil one.” Lawlessness was already in existence in 2 Thessalonians 2, but the lawless one—the Antichrist—was not. He will not be able to gain world authority until he kills three of the ten kings opposing him. So, on the one hand, human government is even now restraining lawlessness. On the other hand, the government of the last of the three kings will restrain the Antichrist, the lawless one, until the middle of the tribulation. Once he kills the third of the last three kings, then the other seven will submit to him, and that is the moment when the lawless one will take over the world and lawlessness will become his government policy.
Our book The Footsteps of the Messiah: A Study of the Sequence of Prophetic Events, has an appendix dealing with 2 Thessalonians 2 that might prove helpful.