The kingdom was offered to Israel but they rejected Jesus as Messiah and King and stoned Stephen. Israel diminished and the dispensation of grace postponed the kingdom.
Note that the Kingdom of God was not postponed, for it is now in heaven (2 Timothy 4:18, Hebrews 12:22-24) and is now on the earth spiritually within Christians (Romans 14:17, Luke 17:21), whether Jews or Gentiles. But in the future the Kingdom will also come fully upon the earth as it is in heaven (Matthew 6:10). It will also be physically (Luke 22:30, Matthew 19:28) on the earth (Revelation 5:10), first during the future Millennium (Revelation 20:4-6, Revelation 2:26-29, Psalms 66:3-4, Psalms 72:8-11, Zechariah 14:3-21), and then forever on the New Earth (Revelation 21:1-8), as in a new surface for the earth.
Jesus Christ's Kingdom is Israel (John 1:49, John 12:13-15, John 19:19, Luke 22:30). And at His future, Second Coming, He will sit on the earthly throne of King David (Luke 1:32-33, Isaiah 9:7) and restore the kingdom to Israel (Acts 1:6-7, Acts 3:20-21). Jesus is, in His humanity, the son of David (Matthew 1:1, Matthew 21:15-16, Romans 1:3), of the house of David (Luke 1:69). So at Jesus' Second Coming, He will restore the tabernacle, the house, of David (Isaiah 16:5, Amos 9:11) to its royal glory (2 Samuel 5:12) which it had lost (2 Kings 17:21a). And Jesus will fulfill the prophecy and prayer of 2 Samuel 7:16-29. And He will bring salvation to all of the still-living, non-Christian elect Jews of the house of David, for they (along with all other still-living, non-Christian elect Jews) will become Christians when they see Jesus at His Second Coming (Zechariah 12:10-14, Zechariah 13:1,6, Romans 11:26-31). And so they will all become part of the Church at that time, for there are no Christians outside of the Church (Ephesians 4:4-6).
After Jesus Christ's future, Second Coming (Revelation 19:7 to 20:3, Zechariah 14:3-5) will occur the Millennium (Revelation 20:4-6, Zechariah 14:8-21), during which time the Gentile nations will come to seek the returned Jesus ruling the whole earth (Zechariah 8:22, Zechariah 14:9, Psalms 72:8-11) on the restored throne of David (Isaiah 9:7) in the earthly Jerusalem (Isaiah 2:1-4, Zechariah 14:8-11,16-19). And the physically resurrected Church will reign on the earth with Jesus during the Millennium (Revelation 20:4-6, Revelation 5:10, Revelation 2:26-29). For the Church is Israel (Romans 11:1,17,24, Ephesians 2:12,19, Galatians 3:29, Revelation 21:9,12; 1 Peter 2:9-10).
Paul turned to the Gentiles with the gospel of grace.
Note that the Gospel of grace was also preached by the other apostles of Jesus Christ:
John 1:16 And of his fulness have all we received, and grace for grace.
17 For the law was given by Moses, but grace and truth came by Jesus Christ.
Acts 4:33 And with great power gave the apostles witness of the resurrection of the Lord Jesus: and great grace was upon them all.
Acts 15:11 . . . we [spoken by Peter] believe that through the grace of the Lord Jesus Christ we shall be saved . . .
1 Peter 5:12 . . . this is the true grace of God wherein ye stand.
1 Peter 5:10 . . . the God of all grace, who hath called us unto his eternal glory by Christ Jesus . . .
1 Peter 4:10 . . . as good stewards of the manifold grace of God.
1 Peter 1:10 Of which salvation the prophets have enquired and searched diligently, who prophesied of the grace that should come unto you . . .
James 4:6 . . . God resisteth the proud, but giveth grace unto the humble.