2 Thessalonians 2:7

For the mystery of iniquity doth already work
Or "the mystery of that wicked one", as the Syriac; meaning either antichrist himself, and the spirit of antichrist, which were already in the world, ( 1 John 2:18 ) ( 4:2 ) , "mystery" being one of the names of antichrist, ( Revelation 17:5 ) and anciently this word was engraven on the mitres of the popes of Rome: or the evil doctrines and practices of antichrist may be intended; for as the doctrine of the Gospel is called a mystery, and the mystery of godliness; so the doctrines and practices of antichrist may be called the mystery of iniquity, especially as they were now secretly spread, imbibed, and practised: the foundations of it were now laying in the church by false teachers; for errors and heresies of every sort, respecting the person and offices of Christ, and in opposition to them, were now broached; idolatry, and holding communion with idolaters, now obtained; worshipping of angels was used by some; and superstition and will worship, worship after the commandments of men, were practised; days, and months, and years, distinguished by Jews and Pagans, and difference of meats, were observed; celibacy and virginity began to be admired and commended; dominion and magistracy were despised, and church authority contemned, and many, as Diotrephes, loved to have the pre-eminence; and the doctrine of justification by the works of the law was industriously spread, and zealously preached and received; all which laid the foundation, and are the life and soul of popery:

only he who now letteth, will let, until he be taken out of the way;
that is, the Roman empire and Roman emperors, and which were by degrees entirely removed, and so made way for the revelation of this wicked one: and which was done partly by Constantine the emperor receiving the Christian faith, whereby the Roman empire as Pagan ceased; and by increasing the riches of the church, and feeding the pride, ambition, and covetousness of the bishops, especially the bishop of Rome; and next by removing the seat of the empire from Rome to Byzantium, which he called Constantinople: here the Greek emperors continued in succession, and neither they themselves, nor even their exarchs, resided at Rome, but at Ravenna; so that way was made for antichrist to come to his seat, and there was nothing to rival and eclipse the grandeur, power, and glory of the Roman popes: and that which let was also taken out of the way, by the division of the empire, by Theodosius, giving to his elder son Arcadius, the eastern, and to the younger, Honorius, the western parts of it: the eastern empire was in process of time seized upon and possessed by Mahomet and the Saracens; and the western empire was overrun by the Goths, Vandals, and Huns, and became extinct about the year 476, in Augustulus, the last of the Roman emperors, who was obliged to abdicate the government by Odoacer king of the Heruli; when the kingdom of the Lombards took place in Italy, and afterwards that was translated to Charles the great, king of the French; so that there was nothing more of the Roman empire remaining than the bare name, as at this day; and by this means the popes of Rome got to the height of their power and glory, which is meant by the revelation of the man of sin.