The Son's subjection is the subjection of his flesh
Marcellus is arguing that the Son's subjection spoken of by Paul belongs to his incarnate flesh, not to his eternal divine kingdom, and he is citing Scripture to show that the Father works through the Son and the Spirit.
For you are God, and we did not know it, the God of Israel, the Savior (Isaiah 45:15). All who oppose him will be put to shame and disgraced. Those oppose him who do not confess that he and his Spirit are of the same being as the Father, and that besides him there is no God, and those who are ashamed of his suffering and his poverty. For no one can know God unless he confesses with the apostle Thomas the one who was crucified and risen from the dead as Lord and God—the one who says to his apostles: “If you had known me, you would have known my Father also; from now on you do know him and have seen him” (John 14:7). Philip says to him, “Lord, show us the Father, and that is enough for us” (John 14:8). Jesus says to him, “Have I been with you so long, and you have not known me, Philip? Whoever has seen me has seen my Father. How can you say, ‘Show us the Father’? Do you not believe that I am in the Father and the Father is in me? The words that I speak I do not speak on my own. The Father who remains in me does his works. Believe me that I am in my Father and the Father is in me; or else believe because of the works themselves” (John 14:9-11). Now it was the work of God the Father to cast out demons, and he said by the Holy Spirit that he cast out demons: “But if I,” he says, “cast out demons by the Spirit of God” (Matthew 12:28). About this Luke says, “But if I cast out demons by the finger of God.” For when Scripture names Christ the arm of the Father, it calls the Holy Spirit the finger of God; and when it names the Son the Word of God, it calls the Holy Spirit the breath of God. And again the apostle says: “When he hands over the kingdom to God the Father, when he abolishes every rule and every authority and power. For he must reign until he has put all his enemies under his feet. The last enemy to be abolished is death. For he has put all things in subjection under his feet. But when it says, ‘All things are put in subjection,’ it is clear that this excludes the one who put all things in subjection to him. When all things are subjected to him, then the Son himself will also be subjected to the one who subjected all things to him, so that God may be all in all” (1 Corinthians 15:24-28). He is speaking about the subjection of the world, the subjection accomplished in his flesh. For about his divine kingdom Daniel declared: “And of his kingdom there will be no end” (Daniel 7:14). Just as Luke says that the angel Gabriel said to the Virgin about the Lord: “And he will reign forever, and of his kingdom there will be no end” (Luke 1:33). But the apostle says his kingdom has an end. “For he must reign,” he says, “until he has put all his enemies under his feet” (1 Corinthians 15:25). In the same way David also says: “The Lord said to my Lord, ‘Sit at my right hand until I make your enemies a footstool for your feet’” (Psalm 110:1).
Greek
Σὺ γὰρ εἶ ὁ Θεὸς, καὶ οὐκ ᾔδειμεν, Θεὸς τοῦ Ἰσραὴλ Σωτήρ. Αἰσχυνθήσονται καὶ ἐντραπήσονται πάντες οἱ ἀντικείμενοι αὐτῷ. Ἀντίκεινται δὲ αὐτῷ οἱ μὴ ὁμολογοῦντες αὐτὸν καὶ τὸ Πνεῦμα αὐτοῦ τῆς αὐτῆς τῷ Πατρὶ οὐσίας, καὶ πλὴν αὐτοῦ μὴ εἶναι Θεὸν, καὶ οἱ τὸ πάθος αὐτοῦ καὶ τὴν πτωχείαν ἐπαισχυνόμενοι. Οὐδεὶς γὰρ δύναται γνῶναι Θεὸν, ἐὰν μὴ τὸν σταυρωθέντα καὶ ἐκ νεκρῶν ἀναστάντα ὁμολογήσῃ σὺν τῷ ἀποστόλῳ Θωμᾷ Κύριον καὶ Θεὸν τὸν λέγοντα τοῖς ἀποστόλοις αὐτοῦ· Εἰ γὰρ ἐγνώκειτέ με, καὶ τὸν Πατέρα μου ἐγνώκειτε ἂν, καὶ ἀπάρτι γινώσκετε αὐτὸν, καὶ ἑωράκατε αὐτόν. Λέγει αὐτῷ Φίλιππος· Κύριε, δεῖξον ἡμῖν τὸν Πατέρα, καὶ ἀρκεῖ ἡμῖν. Λέγει αὐτῷ ὁ Ἰησοῦς· Τοσοῦτον χρόνον μεθ' ὑμῶν εἰμι, καὶ οὐκ ἔγνωκάς με; Φίλιππε, ὁ ἑωρακὼς ἐμὲ ἑώρακε τὸν Πατέρα μου. Καὶ πῶς σὺ λέγεις· Δεῖξον ἡμῖν τὸν Πατέρα; Οὐ πιστεύεις, ὅτι ἐγὼ ἐν τῷ Πατρὶ, καὶ ὁ Πατὴρ ἐν ἐμοί ἐστι; Τὰ ῥήματα ἃ ἐγὼ λαλῶ, οὐκ ἀπ' ἐμαυτοῦ λαλῶ. Ὁ δὲ Πατήρ μου ἐν ἐμοὶ μένων, αὐτὸς ποιεῖ τὰ ἔργα. Πιστεύετέ μοι, ὅτι ἐγὼ ἐν τῷ Πατρί μου, καὶ ὁ Πατὴρ ἐν ἐμοί ἐστιν. Εἰ δὲ μὴ, διὰ τὰ ἔργα αὐτὰ πιστεύετέ μοι. Ἔργον δὲ ἦν τοῦ Θεοῦ Πατρὸς τὸ ἐκβάλλειν τὰ δαιμόνια· ἐν Πνεύματι δὲ ἁγίῳ εἶπεν ἐκβάλλειν τὰ δαιμόνια· Εἰ δὲ ἐγὼ, φησὶν, ἐν Πνεύματι Θεοῦ ἐκβάλλω τὰ δαιμόνια· Περὶ οὗ ὁ Λουκᾶς φησιν· Εἰ δὲ ἐγὼ ἐν δακτύλῳ Θεοῦ ἐκβάλλω 1020 τὰ δαιμόνια. Ὅτε γὰρ ἡ Γραφὴ τὸν Χριστὸν βραχίονα ὀνομάζει τοῦ Πατρὸς, τὸ ἅγιον Πνεῦμα δάκτυλον Θεοῦ καλεῖ· καὶ ὅτε δὲ τὸν Υἱὸν Θεοῦ Λόγον ὀνομάζει, τὸ ἅγιον Πνεῦμα ἐμφύσημα τοῦ Θεοῦ λέγει. Καὶ πάλιν λέγει ὁ Ἀπόστολος· Ὅταν παραδιδῷ τὴν βασιλείαν τῷ Θεῷ καὶ Πατρὶ, ὅταν καταργήσῃ πᾶσαν ἀρχὴν καὶ πᾶσαν ἐξουσίαν καὶ δύναμιν. Δεῖ γὰρ αὐτὸν βασιλεύειν ἄχρις ἂν θῇ πάντας τοὺς ἐχθροὺς ὑπὸ τοὺς πόδας αὐτοῦ. Ἔσχατος ἐχθρὸς καταργεῖται ὁ θάνατος· πάντα γὰρ ὑπέταξεν ὑπὸ τοὺς πόδας αὐτοῦ. Ὅταν δὲ εἴπῃ, ὅτι Πάντα ὑποτέτακται, δῆλον, ὅτι ἐκτὸς τοῦ ὑποτάξαντος αὐτῷ τὰ πάντα. Ὅταν δὲ ὑποταγῇ αὐτῷ τὰ πάντα, τότε καὶ αὐτὸς ὁ Υἱὸς ὑποταγήσεται τῷ ὑποτάξαντι αὐτῷ τὰ πάντα, ἵνα ᾖ ὁ Θεὸς τὰ πάντα ἐν πᾶσι. Περὶ τῆς ὑποταγῆς τοῦ κόσμου λέγει τῆς ἐν σαρκὶ αὐτοῦ ὑποτεταγμένης. Περὶ γὰρ τῆς βασιλείας αὐτοῦ τῆς θεϊκῆς Δανιὴλ ἐξέφρασε λέγων· Καὶ τῆς βασιλείας αὐτοῦ οὐκ ἔσται τέλος. Καθὼς καὶ ὁ Λουκᾶς λέγει τὸν ἄγγελον Γαβριὴλ εἰρηκέναι πρὸς τὴν Παρθένον περὶ τοῦ Κυρίου· Καὶ βασιλεύσει, φησὶν, εἰς τὸν αἰῶνα καὶ τῆς βασιλείας αὐτοῦ οὐκ ἔσται τέλος. Ὁ δὲ Ἀπόστολος λέγει τέλος ἔχειν τὴν βασιλείαν αὐτοῦ. Δεῖ γὰρ αὐτὸν βασιλεύειν, φησὶν, ἄχρις ἂν θῇ πάντας τοὺς ἐχθροὺς ὑπὸ τοὺς πόδας αὐτοῦ. Ὁμοίως δὲ καὶ ὁ Δαβίδ· Εἶπε, φησὶν, ὁ Κύριος τῷ Κυρίῳ μου· Κάθου ἐκ δεξιῶν μου, ἕως ἂν θῶ τοὺς ἐχθρούς σου ὑποπόδιον τῶν ποδῶν σου.