Here are the words of Saint Augustin:
"And who are those who refuse to worship the beast and his image, if not those who observe what the Apostle says: “Do not attach yourselves to the same yoke with the infidels" ? They do not adore him, that is, deny him adhesion and submission; they do not receive his sign, that is, the mark of the crime, neither on the forehead because of the faith they profess, nor on the hand because of their works. […] “From now on indeed the mystery of iniquity is at work” refers only to the wicked and hypocrites who are in the Church until they come to a number strong enough to form a great people to the Antichrist: this is the mystery of iniquity because it seems hidden."