Why did Jesus preach only to Israel for three years?
Jesus preached only to Israel for three years because the covenants required that Israel, the covenant nation, believe first. The gospel had to go to the Jew first, then to the Gentile. Israel, however, would not believe, and that is why the message later turned to the Gentiles.
Les explains that Jesus was the Messiah, the Promised One according to the covenants. For three years He presented Himself only to the nation of Israel because they were the ones who had to believe first. As Les teaches, it had to go to the Jew first, and then to the Gentile, but Israel would not believe. Peter, still hoping against hope that Israel would wake up and realize the One they killed was the Promised Messiah, continued to address them as brethren. At that point, Peter would not address Gentiles as brethren, because they had no idea they were to have anything to do with Gentiles. Later, the message would go to the Gentiles.