The first issue we must recognize is the way God uses the word Gentile in His Word. The context will usually show you how the words are meant to be used. In addition, the scope of the particular section of a book must be accounted for. Before and after Acts 28:28 there are Gentile believers spoken of in the Bible some are called a wild olive branch while others are called The Church which is His Body, Why?
The Apostle Paul's epistles usually use the word Gentile meaning the Gentile believers in the many synagogues. The usage of Gentiles in Romans is especially of interest. Failing to discern this usage in Romans has caused much dismay over many years. It was the Gentile believers, who attended the synagogues, which were grafted into the olive tree, the olive tree being Israel. These were the Gentiles Paul spoke to in his Acts ministry with possibly 3 exceptions.
Taking a look at the scope of Acts, we see that the ministry to the Gentiles was for the benefit of Israel; God was not at that particular time interested in saving the Gentiles. God told Israel, in Romans 10:19, "But, I say, did not Israel know?" First Moses said, "I will provoke you to jealousy by them that are no people, and by a foolish nation I will anger you." Yes, the Gentiles were blesses for Israel's sake. But after Acts the Gentiles were blessed because there was no Israel. They became the center of God's plan, even as Israel had been God's chosen people before now. The purpose of provoking Israel was not made known to them at the first, either by Peter at Cornelius' house, or by Paul at Antioch. Paul tells the Jews in Acts 13:47 that they had been chosen to be a light unto the Gentiles, but since they had rejected their King and His kingdom he turned to the Gentiles. He did not say here that the salvation of God was sent to the Gentiles. Salvation was still for the Jews (John 4:22), and had not yet been sent to the Gentiles (Acts 28:28).
This shows us why Paul wrote in Acts 26:22, 23 that in all he did from the beginning of his ministry until that time, he has said no other thing than those things which Moses and the prophets said. This was all the will of God not the Apostles choice or his own ministry. Now what did Moses and the prophets say? That Christ should suffer, be the firstborn from the dead and show light to Israel and the Gentiles. This is not the mystery which had been hid in God from ages and generations. Paul could quote chapter and verse from the Jewish Bible to prove what he said.
From the beginning of Acts the twelve Apostles continued what the Lord both began to do and to say. Paul simply continued that same ministry; for it was what he saw (Acts 26:16). Throughout Acts Paul preached what he had seen and heard the twelve Apostles preaching. That was his theme also. The Apostle Paul needed a vision concerning the scope of that ministry. Peter had the same experience. Peter was told to go preach to the house of Cornelius. Peter did not change his gospel he just preached it to somebody else, the Gentiles. So it was with Paul. He preached his gospel of the King and kingdom to the Jews first, and then to the Gentiles. Paul did not require the Gentiles to be circumcised and keep the law. They were blessed with Israel thru Abraham by grace without the addition of the law. This was Paul's gospel of the grace of God. But that ended in Acts 28:28. Acts 28:28 declares the salvation of God was sent to the Gentiles, and this salvation was not the blessings of the kingdom, but rather the Church with Christ as Head.
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