through the hypocrisy of men who speak lies, branded in their own conscience as with a hot iron;
But there is no turning back for the hypocritical liars Paul talks about because they have a conscience that is seared with a hot iron. They have not maintained a conscience void of offense but have so consistently violated it that it is "past feeling" as the stump of a severed extremity after being seared with a hot iron has no feeling.