Or, “branded with a branding iron.” Some hold that this refers to the insensibility of a conscience that is no longer aware of guilt at wrongdoing in somewhat the same way that searing by a hot iron renders human flesh incapable of feeling, so that it becomes progressively difficult for the Holy Spirit to make any impression on the conscience. Compare the course of Judas, who finally ignored every appeal to his soul (see Luke 22:3; John 6:70; 13:27). Others believe that as a branding iron leaves its mark, so the “doctrines of devils” (1 Tim. 4:1) and hypocritical “lies” (v. 2) make the satanic brand an indelible mark. Hence, as Paul possessed the “marks” of his service to Christ (Gal. 6:17), so these deceivers will bear corresponding marks of loyalty to Satan.

 Or, “pretense,” “outward show.” Teachers of deception (v. 1) may pretend loyalty to truth while they spread their “doctrines of devils” (v. 1). Apostates often do not sail openly under the flag of error and treason to the cause of Christ. The teachers of deception trumpet loudly their loyalty to the cause of truth, thus better to delude men.

Literally, “their own conscience.” The deceptions that are misleading others in the church are, at the same time, making the deceivers insensitive to truth.