J. B. Lightfoot on Colossians 2:9-10

πεπληρωμένοι, ὅς ἐστιν ἡ κεφαλὴ πάσης ἀρχῆς καὶ πεπληρωμένοι] ‘_being fulfilled_’, with a direct reference to the preceding πλήρωμα; ‘Your fulness comes from His fulness; His πλήρωμα is transfused into you by virtue of your incorporation in Him’. So too John i. 16 ἐκ τοῦ πληρώματος αὐτοῦ ἡμεῖς πάντες ἐλάβομεν, Ephes. iii. 19 ἵνα πληρωθῆτε εἰς πᾶν τὸ πλήρωμα τοῦ Θεοῦ, iv. 13 εἰς μέτρον ἡλικίας τοῦ πληρώματος τοῦ Χριστοῦ, comp. Ign. _Ephes._ init. τῇ εὐλογημένῃ ἐν μεγέθει Θεοῦ πατρὸς πληρώματι. Hence also the Church, as ideally regarded, is called the πλήρωμα of Christ, because all His graces and energies are communicated to her; Ephes. i. 23 ἥτις ἐστὶν τὸ σῶμα αὐτοῦ, τὸ πλήρωμα τοῦ τὰ πάντα ἐν πᾶσιν πληρουμένου. ὅς] For the various reading ὅ see the detached note. It was perhaps a correction made on the false supposition that ἐν αὐτῷ referred to the πλήρωμα. At all events it must be regarded as an impossible reading; for the image would be altogether confused and lost, if the πλήρωμα were represented as the head. And again ἡ κεφαλὴ is persistently said elsewhere of Christ; i. 18, ii. 19, Ephes. i. 22, iv. 15, v. 23. Hilary _de Trin._ ix. 8 (II. p. 264) explains the ὅ as referring to the whole sentence τὸ εἶναι ἐν αὐτῷ πεπληρωμένους, but this also is an inconceivable sense. Again it has been suggested that ὅ ἐστιν (like τουτέστιν) may be taken as equivalent to _scilicet_ (comp. Clem. _Hom._ viii. 22); but this would require τῇ κεφαλῇ, even if it were otherwise admissible here.