Rom 1:16 For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek.
Why was the Jews given the gospel first and not the Gentiles or at least at the same time?
Mat 10:5-7 "These twelve Jesus sent out and commanded them, saying: "Do not go into the way of the Gentiles, and do not enter a city of the Samaritans. But go rather to the lost sheep of the house of Israel. And as you go, preach, saying, 'The kingdom of heaven is at hand.'"
Our Lord had a humble birth amongst animals, but He was strictly brought up a Jews - was circumcised on the eighth day, and started His ministry at 30 years old.
He was sent to preach only to Israelites, and when He sent His disciples out, He commanded them to preach only to Israelites. At that time most of the Jews were hypocrites - took Judaism as only laws and a religion lacking justice, mercy and faith in God. (Please read Mat 23)
Matthew 23:23 "Woe to you, scribes and Pharisees, hypocrites! For you pay tithe of mint and anise and cummin, and have neglected the weightier matters of the law: justice and mercy and faith. These you ought to have done, without leaving the others undone."
Mat 23:15 "Woe to you, scribes and Pharisees, hypocrites! For you travel land and sea to win one proselyte, and when he is won, you make him twice as much a son of hell as yourselves."
Had any Gentile become a Christian then, he would be like the Jewish hypocrites with their destination in Hell.
Matthew 23:33, 36 "Serpents, brood of vipers! How can you escape the condemnation of hell?...Assuredly, I say to you, all these things will come upon this generation."
Samaritan Jews were Saved
Acts 8:4-6 Therefore those who were scattered went everywhere preaching the word. Then Philip went down to the city of Samaria and preached Christ to them. And the multitudes with one accord heeded the things spoken by Philip, hearing and seeing the miracles which he did.
At Pentecost God baptized His Apostles with the Holy Spirit of truth. Only after He had sacrificed Stephen then the disciples out of fear of Saul's persecution, scattered and preached the gospel. In Samaria many Jews were saved.
After Paul was converted, there were peace and revival in all the churches.
Acts 9:31 Then the churches throughout all Judea, Galilee, and Samaria had peace and were edified. And walking in the fear of the Lord and in the comfort of the Holy Spirit, they were multiplied.
God sent Peter to the Gentiles
Acts 10:10-15 Then he became very hungry and wanted to eat; but while they made ready, he fell into a trance and saw heaven opened and an object like a great sheet bound at the four corners, descending to him and let down to the earth. In it were all kinds of four-footed animals of the earth, wild beasts, creeping things, and birds of the air. And a voice came to him, "Rise, Peter; kill and eat." But Peter said, "Not so, Lord! For I have never eaten anything common or unclean." And a voice spoke to him again the second time, "What God has cleansed you must not call common."
All His Apostles and disciples who were Jews before, refused to mix with Gentiles. With signs and wonders, our Lord Jesus convinced Peter to go to the Gentiles.
Acts 10:19-20 While Peter thought about the vision, the Spirit said to him, "Behold, three men are seeking you. Arise therefore, go down and go with them, doubting nothing; for I have sent them."