ὁ καταβὰς αὐτός ἐστιν καὶ ὁ ἀναβὰς ὑπεράνω πάντων τῶν οὐρανῶν ἵνα πληρώσῃ τὰ πάντα. The one who descended himself is also the one who ascended above all the heavens in order that he would fill all things.
Paul continues his argument from verse 9 here in verse 10. He draws a conclusion that the one who descended is also the one who ascended. The inference is made that one cannot ascend without first descending if the one belongs to a higher region. Paul is saying that Christ descended from the heavens, came down to earth, and then ascended back to the heavens.
In the beginning of this sentence, Paul uses a reflexive intensive formulation to refer back to Christ: ὁ καταβὰς αὐτός (“the one who descended himself”). This formulation seems awkward to us, but it is pointing the listeners and readers back to Christ as the subject. Christ is the one who descended, and he is also the one who ascended.
But why did Christ descend and ascend? It was for this purpose, that he would fill all things. This idea of filling all things is linked with Ephesians 1:23. Christ is the ruler over the church and over all things. All things are subject to Christ. Christ has fulfilled his role, God placed him in authority over all things, and therefore he fills all things.
The one who descended himself is also the one who ascended above all the heavens, for the purpose that he would fill all things.
Since Christ has come to this earth and ascended back into heaven, he has been placed in authority over all things. Nothing escapes his reign. We can place our hope and our confidence in Christ precisely because of what he has done and what God has given to him. He came to this earth and made salvation possible, but he also ascended back into the heavens where God placed him in authority over all things, not simply the church universal. With Christ in charge, what have we to fear?