Cyclopedia of Biblical, Theological and Ecclesiastical Literature
Mystery of Iniquity
(τὸ μυστήριον τῆς ἀνομίας ), an expression that occurs in Paul's description of the workings of an antichristian power in his own day (1 Thessalonians 2:7), and the meaning of which is not clear. The attributive genitive (ἀνομίας ) does not seem to be that of the agent (Theodoret), nor that of apposition (Lunemann and Alford), but simply of definition, or of the characterizing quality, i.e., the mystery of which the characterizing feature, or the active principle, was ἀνομία, or lawlessness — the antithesis of order and legality. This "mystery of iniquity" was no personality, i.e., Antichrist, or any real or assumed type of Antichrist (as Chrysostom), but all that mass of uncombined and, so to speak, unorganized lawlessness which, though as yet seen only in detail and not revealed in its true proportions, was even then (ἤδη) aggregating and energizing, and would eventually (ἐν τῷ ἑαυτοῦ καιρῷ ) find its complete development and organization in the person and power of Antichrist (Ellicott, note ad loc.). (See ANTICHRIST).
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McClintock, John. Strong, James. Entry for 'Mystery of Iniquity'. Cyclopedia of Biblical, Theological and Ecclesiastical Literature. http://www.studylight.org/encyclopedias/tce/m/mystery-of-iniquity.html. Harper & Brothers. New York. 1870.