In Eph. 1:10, the Greek text according to the Textus Receptus states,
Ιʹ εἰς οἰκονομίαν τοῦ πληρώματος τῶν καιρῶν ἀνακεφαλαιώσασθαι τὰ πάντα ἐν τῷ Χριστῷ τὰ τε ἐν τοῖς οὐρανοῖς καὶ τὰ ἐπὶ τῆς γῆς TR, 1550
According to my understanding, the idea is that everything in heaven and upon earth is recapitulated (from the lemma ἀνακεφαλαιῶ), that is, brought under a single head, “in Christ.” Although it has a few meanings, I understand “in Christ” here to mean, “by being a Christian and being incorporated into the body of Christ.” That being said, I can understand what “everything that/everyone who is upon the earth” («τὰ πάντα…τὰ ἐπὶ τῆς γῆς») refers to, which would be Christians alive on earth (i.e., the Church militant).
However, to what or whom does the phrase “everything that/everyone who is in heaven” («τὰ πάντα…τὰ…ἐν τοῖς οὐρανοῖς») refer? Do the other verses in the epistle shed any light on its meaning or referent?