In the first pericope of Luke 9, Jesus gives his Apostles their marching orders, and the ring composition with variation tells us something interesting about their mission.
Συγκαλεσάμενος δὲ τοὺς δώδεκα ἔδωκεν αὐτοῖς δύναμιν καὶ ἐξουσίαν ἐπὶ πάντα τὰ δαιμόνια καὶ νόσους θεραπεύειν, 2 καὶ ἀπέστειλεν αὐτοὺς κηρύσσειν τὴν βασιλείαν τοῦ θεοῦ καὶ ἰᾶσθαι [τοὺς ἀσθενεῖς], 3 καὶ εἶπεν πρὸς αὐτούς· Μηδὲν αἴρετε εἰς τὴν ὁδόν, μήτε ῥάβδον μήτε πήραν μήτε ἄρτον μήτε ἀργύριον, μήτε ἀνὰ δύο χιτῶνας ἔχειν. 4 καὶ εἰς ἣν ἂν οἰκίαν εἰσέλθητε, ἐκεῖ μένετε καὶ ἐκεῖθεν ἐξέρχεσθε. 5 καὶ ὅσοι έχωνται ὑμᾶς, ἐξερχόμενοι ἀπὸ τῆς πόλεωςἐκείνης τὸν κονιορτὸν ἀπὸ τῶν ποδῶν ὑμῶν ἀποτινάσσετε εἰς μαρτύριον ἐπ’ αὐτούς. 6 ἐξερχόμενοι δὲ διήρχοντο κατὰ τὰς κώμας εὐαγγελιζόμενοι καὶ θεραπεύοντες πανταχοῦ.
And he called the twelve together and gave them power and authority over all demons and to cure diseases, 2 and he sent them out to proclaim the kingdom of God and to heal. 3 And he said to them,“Take nothing for your journey, no staff, nor bag, nor bread, nor money; and do not have two tunics. 4 And whatever house you enter, stay there, and from there depart. 5 And wherever they do not receive you, when you leave that town shake off the dust from your feet as a testimony against them.” 6 And they departed and went through the villages, preaching the gospel and healing everywhere.
The “good news” that they preached was the “kingdom of God” (Matthew in the parallel passage calls it the “kingdom of heaven”): this is expressed by substituting εὐαγγελιζόμενοι (“preaching good news”) for κηρύσσειν τὴν βασιλείαν τοῦ θεοῦ (“to herald the kingdom of God”). The miracles they worked served as confirmation of their message, as testimony to the good news of God’s rule over the world; and they also were a demonstration of it. Because the Twelve were acting as servants of a king and of a kingdom, Christ endowed them with kingly attributes to do their work that they did not have of themselves: 1 δύναμις καὶ ἐξουσία.
From this early stage, the kingdom of God was to expand far beyond Judea to cover the whole world. Thus Calvin on Matthew 10:1:
At a future period, God extended far beyond Zion the scepter of the power of his Son, and caused rivers to flow from that fountain, to water abundantly the four quarters of the world. Then God assembled his Israel from every direction, and united into one body not only the scattered and torn members, but men who had formerly been entirely alienated from the people of God.