That in the dispensation, etc. (εἰς οἰκονομίαν)
The A.V. is faulty and clumsy. Εἱς does not mean in, but unto, with a view to. Dispensation has no article. The clause is directly connected with the preceding: the mystery which He purposed in Himself unto a dispensation. For οἰκονομία dispensation see on Col 1:25. Here and Eph 3:2, of the divine regulation, disposition, economy of things.
Of the fullness of times (τοῦ πληρώματος τῶν καιρῶν)
For fullness, see on Rom 11:12; see on Joh 1:16; see on Col 1:19. For times, compare Gal 4:4, “fullness of the time (τοῦ χρόνου), where the time before Christ is conceived as a unit. Here the conception is of a series of epochs. The fullness of the times is the moment when the successive ages of the gospel dispensation are completed. The meaning of the whole phrase, then, is: a dispensation characterized: by the fullness of the times: set forth when the times are full.
To sum up all things in Christ (ἀνακεφαλαιώσασθαι)
Explanatory of the preceding phrase; showing in what the dispensation consists. For the word, see on Rom 13:9. It means to bring back to and gather round the main point (κεφαλαίον), not the head (κεφαλή); so that, in itself, it does not indicate Christ (the Read) as the central point of regathering, though He is so in fact. That is expressed by the following in Christ. The compounded preposition ἀνά signifies again, pointing back to a previous condition where no separation existed. All things. All created beings and things; not limited to intelligent beings. Compare Rom 8:21; 1Co 15:28.
The connection of the whole is as follows: God made known the mystery of His will, the plan of redemption, according to His own good pleasure, in order to bring to pass an economy peculiar to that point of time when the ages of the christian dispensation should be fulfilled - an economy which should be characterized by the regathering of all things round one point, Christ.
God contemplates a regathering, a restoration to that former condition when all things were in perfect unity, and normally combined to serve God's ends. This unity was broken by the introduction of sin. Man's fall involved the unintelligent creation (Rom 8:20). The mystery of God's will includes the restoration of this unity in and through Christ; one kingdom on earth and in heaven - a new heaven and a new earth in which shall dwell righteousness, and “the creation shall be delivered from the bondage of corruption into the liberty of the glory of the children of God.”