I can't think of a more succinct explanation of the entire gospel of the kingdom of God than this statement by Peter - shortly after the Day of Pentecost.

Repent, then, and turn to God, so that your sins may be wiped out, that times of refreshing may come from the Lord, and that he may send the Messiah, who has been appointed for you—even Jesus. Heaven must receive Him until the time comes for God to restore everything, as He promised long ago through His holy prophets.

Acts 3:19-21