The Body pf Christ is the restrainer due to the Holy Spirit in each believer.
Well that is true in the corporate sense of the church...but the Holy Spirit is his own person and “goes where he wills” “like the wind blows” as Jesus said. What was it that kept Satan restrained, thus keeping the son of Perdition in check before there even was a “church” or believers in Christ? That’s why this notion of a “restrainer” I think is a bit more historically complicated than believing it was simply is the influence of Spirit filled believers that keeps the Son of Perdition in check. Something was keeping him in check before Jesus came onto the scene and even when he was, Christ himself said that the spirit of antichrist was already extant in the world. Satan himself made a play for Jesus, tempting him with all the Kingdoms of the world if only Jesus would fall down and worship him.
The Restrainer could be removed yet the Spirit of God could still reside with believers, even if we are not taken off the Earth for a length of time, perhaps even in prep for our rapture, a time of final testing and faith building, a time to become as the church of Philadelphia noted in Revelation.(Our own corporate Gethsamanee, only as the Bible says, we who are alive are raptured to meet him ‘in the middle of the air” to escape those tribulational horrors as Christ promised his elect that they should escape( in Matthew).