J. B. Lightfoot on Colossians 1:25-26

οἰκονομίαν τοῦ Θεοῦ τὴν δοθεῖσάν μοι εἰς ὑμᾶς, πληρῶσαι τὸν λόγον τοῦ Θεοῦ, ^{26} τὸ μυστήριον τὸ ἀποκεκρυμμένον εἰς ὑμᾶς] ‘_to youward_,’ i.e. ‘for the benefit of you, the Gentiles’; εἰς ὑμᾶς being connected with τὴν δοθεῖσάν μοι, as in Ephes. iii. 2 τὴν οἰκονομίαν τῆς χάριτος τοῦ Θεοῦ τῆς δοθείσης μοι εἰς ὑμᾶς; comp. Rom. xv. 16 διὰ τὴν χάριν τὴν δοθεῖσάν μοι ὑπὸ τοῦ Θεοῦ εἰς τὸ εἶναί με λειτουργὸν Χριστοῦ Ἰησοῦ _εἰς τὰ ἔθνη_. πληρῶσαι] ‘_to fulfil_,’ i.e. ‘to preach fully,’ ‘to give its complete development to’; as Rom. xv. 19 ὥστε με ἀπὸ Ἰερουσαλὴμ καὶ κύκλῳ μέχρι τοῦ Ἰλλυρικοῦ πεπληρωκέναι τὸ εὐαγγελίον τοῦ Χριστοῦ. Thus ‘the word of God’ here is ‘the Gospel,’ as in most places (1 Cor. xiv. 36, 2 Cor. ii. 17, iv. 2, etc.), though not always (e.g. Rom. ix. 6), in St Paul, as also in the Acts. The other interpretation, ‘to accomplish the promise of God,’ though suggested by such passages as 1 Kings ii. 27 πληρωθῆναι τὸ ῥῆμα Κυρίου, 2 Chron. xxxvi. 21 πληρωθῆναι λόγον Κυρίου, etc., is alien to the context here. 26. τὸ μυστήριον] This is not the only term borrowed from the ancient mysteries, which St Paul employs to describe the teaching of the Gospel. The word τέλειον just below, ver. 28, seems to be an extension of the same metaphor. In Phil. iv. 12 again we have the verb μεμύημαι: and in Ephes. i. 14 σφραγίζεσθαι is perhaps an image derived from the same source. So too the Ephesians are addressed as Παύλου συμμύσται in Ign. _Ephes._ 12. The Christian teacher is thus regarded as a ἱεροφάντης (see Epict. iii. 21. 13 sq.) who initiates his disciples into the rites. There is this difference however; that, whereas the heathen mysteries were strictly confined to a narrow circle, the Christian mysteries are freely communicated to all. There is therefore an intentional paradox in the employment of the image by St Paul. See the notes on πάντα ἄνθρωπον τέλειον below. Thus the idea of _secresy_ or _reserve_ disappears when μυστήριον is adopted into the Christian vocabulary by St Paul: and the word signifies simply ‘a truth which was once hidden but now is revealed,’ ‘a truth which without special revelation would have been unknown.’ Of the nature of the truth itself the word says nothing. It may be transcendental, incomprehensible, mystical, mysterious, in the modern sense of the term (1 Cor. xv. 51, Eph. v. 32): but this idea is quite accidental, and must be gathered from the special circumstances of the case, for it cannot be inferred from the word itself. Hence μυστήριον is almost universally found in connexion with words denoting revelation or publication; e.g. ἀποκαλύπτειν, ἀποκάλυψις, Rom. xvi. 25, Ephes. iii. 3, 5, 2 Thess. ii. 7; γνωρίζειν Rom. xvi. 26, Ephes. i. 9, iii. 3, 10, vi. 19; φανεροῦν Col. iv. 3, Rom. xvi. 26, 1 Tim. iii. 16; λαλεῖν iv. 3, 1 Cor. ii. 7, xiv. 2; λέγειν, 1 Cor. xv. 51. But the one special ‘mystery’ which absorbs St Paul’s thoughts in the Epistles to the Colossians and Ephesians is the free admission of the Gentiles on equal terms to the privileges of the covenant. For this he is a prisoner; this he is bound to proclaim fearlessly (iv. 3, Ephes. vi. 19); this, though hidden from all time, was communicated to him by a special revelation (Ephes. iii. 3 sq.); in this had God most signally displayed the lavish wealth of His goodness (ver. 27, ii. 2 sq., Ephes. i. 6 sq., iii. 8 sq.). In one passage only throughout these two epistles is μυστήριον applied to anything else, Ephes. v. 32. The same idea of the μυστήριον appears very prominently also in the thanksgiving (added apparently later than the rest of the letter) at the end of the Epistle to the Romans, xvi. 25 sq. μυστηρίου ... εἰς ὑπακοὴν πίστεως εἰς πάντα τὰ ἔθνη γνωρισθέντος.