The Lord Jesus preached about the Gospel of the Kingdom of God, a Kingdom on Earth, where He would be King and rule with them.
When He ascended into Heaven, He told them until He returned, they were to continue with this same message (Acts 1:7-8)
Acts 1:2 Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen:
Acts 2:42 And they continued stedfastly in the apostles’ doctrine and fellowship, and in breaking of bread, and in prayers.
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.
2 Peter 3:2 That ye may be mindful of the words which were spoken before by the holy prophets, and of the commandment of us the apostles of the Lord and Saviour:
WHO preached what gospel?
Acts 8:25 And they, when they had testified and preached the word of the Lord, returned to Jerusalem, and preached the gospel in many villages of the Samaritans.
Acts 14:7 And there they preached the gospel.
Acts 14:21 And when they had preached the gospel to that city, and had taught many, they returned again to Lystra, and to Iconium, and Antioch,
Acts 15:7 And when there had been much disputing, Peter rose up, and said unto them, Men and brethren, ye know how that a good while ago God made choice among us, that the Gentiles by my mouth should hear the word of the gospel, and believe.
The Apostle Paul
Acts 16:10 And after he had seen the vision, immediately we endeavoured to go into Macedonia, assuredly gathering that the Lord had called us for to preach the gospel unto them.
Acts 20:24 But none of these things move me, neither count I my life dear unto myself, so that I might finish my course with joy, and the ministry, which I have received of the Lord Jesus, to testify the gospel of the grace of God.
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⇛ also see Doctrines of Paul