"Letteth" means to hinder and "he" refers to the "man of sin" who's mentioned in previous verses. So, the verse can be written like this, "For the mystery of iniquity doth already work: only the man of sin who now hinders will hinder, until the man of sin be taken out of the way." So, the man of sin is hindering and is being "taken out of the way" when he's just been revealed at the temple? How? Why? Perhaps it's referring to the MORTAL man of sin who gets killed and then is 'resurrected' as the Antichrist? See also the note on Revelation 13:3 and Revelation 17:8.