J. B. Lightfoot on Colossians 2:6-7

^7ἐρριζωμένοι καὶ ἐποικοδομούμενοι ἐν αὐτῷ καὶ βεβαιούμενοι τῇ πίστει, καθὼς ἐδιδάχθητε, περισσεύοντες ἐν αὐτῇ ἐν εὐχαριστίᾳ. ἐποικοδομούμενοι] ‘_being built up_,’ as in 1 Cor. iii. 10–14. After this verb we might have expected ἐπ’ αὐτῷ or ἐπ’ αὐτόν (1 Cor. iii. 12) rather than ἐν αὐτῷ; but in this and the companion epistle Christ is represented rather as the binding element than as the foundation of the building: e.g. Ephes. ii. 20 ἐποικοδομηθέντες ἐπὶ τῷ θεμελίῳ τῶν ἀποστόλων καὶ προφητῶν, ὄντος _ἀκρογωνιαίου_ αὐτοῦ Χριστοῦ Ἰησοῦ, _ἐν_ ᾧ πᾶσα [ἡ] οἰκοδομὴ αὔξει εἰς ναὸν ἅγιον _ἐν_ Κυρίῳ, _ἐν_ ᾧ καὶ ὑμεῖς συνοικοδομεῖσθε. The ἐπὶ in ἐποικοδομεῖν does not necessarily refer to the original foundation, but may point to the continued progress of the building by successive layers, as e.g. [Aristot.] _Rhet. ad Alex._ 4 (p. 1426) ἐποικοδομοῦντα τὸ ἕτερον ὡς ἐπὶ τὸ ἕτερον αὔξειν. Hence ἐποικοδομεῖν is frequently used absolutely, ‘_to build up_’ (e.g. Jude 20, Polyb. iii. 27, 4), as here. The repetition of ἐν αὐτῷ emphasizes the main idea of the passage, and indeed of the whole epistle. τῇ πίστει] ‘_by your faith_’, the dative of the instrument; comp. Heb. xiii. 9 καλὸν γὰρ χάριτι βεβαιοῦσθαι τὴν καρδίαν. Faith is, as it were, the cement of the building: comp. Clem. Rom. 22 ταῦτα πάντα βεβαιοῖ ἡ ἐν Χριστῷ πίστις. καθὼς ἐδιδάχθητε] i.e. ‘remaining true to the lessons which you received from Epaphras, and not led astray by any later pretenders’: comp. i. 6, 7 ἐν ἀληθείᾳ, καθὼς ἐμάθετε ἀπὸ Ἐπαφρᾶ. ἐν αὐτῇ κ.τ.λ.] The same ending occurs in iv. 2. Thanksgiving is the end of all human conduct, whether exhibited in words or in works. For the stress laid on thanksgiving in St Paul’s epistles generally, see the note on Phil. iv. 6. The words εὐχάριστος, εὐχαριστεῖν, εὐχαριστία, occur in St Paul’s writings alone of the Apostolic epistles. In this epistle especially the duty of thanksgiving assumes a peculiar prominence by being made a refrain, as here and in iii. 15, 17, iv. 2: see also i. 12.