John the Baptist preached for people to repent for the kingdom of HEAVEN was at hand, Mt 3:2. The Gospel of Mark says John the Baptist was preaching about the kingdom of GOD, Mk 1:15. In other words, the terms are interchangeable. Matthew nearly always calls it kingdom of HEAVEN. Mark on the other hand, in 20 or so places refers to the same kingdom as the kingdom of GOD (e.g., compare the parables in Matthew 13 and Mark 4).
“Jesus answered, “My kingdom is not of this world. If My kingdom were of this world, then My servants would be fighting so that I would not be handed over to the Jews; but as it is, My kingdom is not of this realm.” – Jn 18:36
Acts 2 shows that Jesus ascended to heaven to the right hand of God to be seated on the throne of David, Acts 2. He was killed “king of the Jews”. Pilate answered correctly, “so you are a king.” But Jesus said that His kingdom is “not of this world”, meaning the kingdom was spiritual in nature. Because the “kingdom of heaven is in you”, Hearing about the resurrected and reigning Christ, Luke says the people were pricked in their hearts. They were told to repent and be baptized and they would be forgiven. On the same day about 3,000 souls received the words and got baptized. We conclude that they were added to the kingdom.
This Kingdom was part and parcel to the Good News of the Gospel of Christ. Matthew says they went about preaching the good news of the kingdom, Mt 9:35; 24:14. This is similar to the preaching we find by Philip in Acts which says, “But when they believed Philip preaching the good news about the kingdom of God and the name of Jesus Christ, they were being baptized, men and women alike.” Acts 8:12. If you have heard the true Gospel of Christ, then you have heard the good news of His Kingdom.
“He delivered us out of the domain of darkness and transferred us into the kingdom of His beloved Son” – Col 1:13.
If you are a disciple of Jesus Christ, then you have submitted to “the King of kings, and the Lord of lords” (1 Timothy 6:15, Revelation 17:14, and Revelation 19:16). If you have heard the Gospel of the kingdom of God and have been baptized (Acts 8:12), then you are in His kingdom.
“He has made us to be a kingdom, priests to His God and Father—to Him be the glory and the dominion forever and ever. Amen.” – Rev 1:5; cf Rev 5:10.