According to the Synoptic Gospels, the apostles had announced the good news of the kingdom of God
even before they understood that Jesus must die and rise again. By word and miraculous sign, they had proclaimed the truly glad tidings that Jesus had come to free Israel from death and from her demonic enemies and to preach good news to the poor, thereby fulfilling Messianic prophecy
. Jesus' work to destroy Satan's kingdom continued as he died, rose, and ascended to heaven. The Son of the Virgin Mary now reigns with all power on heaven and earth.
St. Luke is particularly clear about the content of "the gospel of the kingdom." Jesus and the apostles heralded the gospel, the good news of the kingdom (Luke 4:43; 8:1; 16:16; Acts 8:12). Jesus was anointed as Messiah to announce to the poor the good news of liberty and healing (Luke 4:18-19, 43):
The Spirit of the Lord is upon me, because he has anointed me to proclaim good news to the poor. He has sent me to proclaim liberty to the captives and recovering of sight to the blind, to set at liberty those who are oppressed, to proclaim the year of the Lord's favor.
Jesus proclaimed to the poor this good news of freedom from the curse (Luke 4:20):
Blessed are you poor,
For yours is the kingdom of God.
Jesus and the disciples not only proclaimed the good news of the kingdom, but actually brought the glad tidings of the kingdom as he healed the sick and cast out demons (Luke 7:22; 8:1-3; 10:9-11). By attending to that message rather than the things of this age, Mary of Bethany received the one thing needed, which would not be taken away from her (Luke 10:38-42). The very Consolation of Israel, One greater than Solomon, long foretold by the law and prophets, had come! His reign means the deliverance from all of the enemies of God’s people!
The message of Jesus to be heralded to the nations in the power of the Spirit was the message of repentance and the forgiveness of sins (Luke 24:47-49). The fact that the main way the Spirit works in Luke-Acts is by empowering bold proclamation of that message means he brings the kingdom through the forgiveness of sins. That is as hard as making a lame man whole (Luke 5:23 = Mark 2:9 = Matthew 9:5): Jesus announced that message by the power of the Spirit (Luke 3:22; 4:1, 14, 18), just as he brought the kingdom through exorcism in the power of the Spirit (Luke 11:20 with Matthew 12:28). Indeed, he was rejected at Nazareth because he claimed to be the one who was empowered by the Spirit to heal the sick and to proclaim the gospel to the poor (Luke 4:18-21); doing those things showed John's disciples that Jesus was the Messiah (Luke 7:22 = Matthew 11:4-5). The coming of the Spirit to Gentiles showed that they, too, should receive the baptism of forgiveness (Acts 10:47-48). Baptism in Luke-Acts is cleansing for repentance and forgiveness (Acts 2:38; 22:16) and thus is included in the message of repentance and forgiveness of sins to be proclaimed to the nations (Luke 24:47); those who believed the good news of the kingdom were baptized (Acts 8:12).
Just as healing was performed in Jesus' name, forgiveness through faith in his name was proclaimed (Luke 24:47; Acts 2:38; 3:6; 3:16; 4:10-12; 10:43; 16:18). For through his disciples acting in his name, Jesus himself continues to heal and to proclaim the good news (Acts 9:34; 26:22-23). Jesus called sinners to repentance from the beginning of his ministry (Luke 5:31-32; 19:10), and the risen Lord still blesses sinners by turning them from their iniquities (Acts 3:26)—repentance as well as forgiveness is a gift from the now-exalted Prince (Acts 5:31).
The good news of salvation from sin and its curse is encapsulated in forgiveness through faith in Jesus’ name. This is "the good news of peace" (Acts 10:36-43; cf. 17:30-31):
God anointed Jesus of Nazareth with the Holy Spirit and with power. He went about doing good and healing all who were oppressed by the devil, for God was with him. And we are witnesses of all that he did both in the country of the Jews and in Jerusalem. They put him to death by hanging him on a tree, but God raised him on the third day and made him to appear, not to all the people but to us who had been chosen by God as witnesses, who ate and drank with him after he rose from the dead. And he commanded us to preach to the people and to testify that he is the one appointed by God to be judge of the living and the dead. To him all the prophets bear witness that everyone who believes in him receives forgiveness of sins through his name.
Paul, according to Luke, gives examples of how “all the prophets” bear witness of that (Acts 13:32-39):
And we bring you the good news that what God promised to the fathers, this he has fulfilled to us their children by raising Jesus, as also it is written in the second Psalm, "'You are my Son, today I have begotten you.' And as for the fact that he raised him from the dead, no more to return to corruption, he has spoken in this way, "'I will give you the holy and sure blessings of David.' Therefore he says also in another psalm, "'You will not let your Holy One see corruption.' For David, after he had served the purpose of God in his own generation, fell asleep and was laid with his fathers and saw corruption, but he whom God raised up did not see corruption. Let it be known to you therefore, brothers, that through this man forgiveness of sins is proclaimed to you, and by him everyone who believes is freed from everything from which you could not be freed by the law of Moses.
Rejoice, believing the forgiveness of sins that has been proclaimed to you!