The Man of Sin in the Temple of God

Paul tells us in the second letter to the Thessalonians that Christ will not return until there is a falling away and the man of sin sits in the temple of God, claiming to be God. What is meant by “falling away,” and what is the future of those who fall away? Who is “restraining” the return of Christ, and for how long, and who is the man of sin? Many misunderstand his identity. The Bible also shows us that there will be a literal temple in Jerusalem before Christ’s Second Coming, but why is it being built at all? And why is it called the “temple of God”?