through the hypocrisy of men who speak lies, branded in their own conscience as with a hot iron;
The meaning then is that, like actors on the ancient Greek stage who assumed a mask in order to portray a character different from themselves, liars must make themselves appear different from their true character. In this instance, it may be that because the demons have deceived them they are unaware of their hypocrisy. On the other hand, it may be that they are perfectly conscious of it. At any rate, lying has become second nature to them because they have abandoned the standard of God’s word. They feel no compunction because their consciences have been seared. As in Ephesians 4:19, they are "past feeling."