Why did Jesus teach only the nation of Israel and not the Gentiles?
Jesus taught only Israel because the covenant promises required Israel to believe first. For three years He presented Himself as the promised Messiah exclusively to the nation of Israel. The gospel had to go to the Jew first and then to the Gentile, but Israel would not believe.
Les explains that Jesus was the Messiah, the Promised One according to the covenants. For three years He tried to get across to the nation of Israel, and only to Israel, because they were the ones who had to believe first. The order was to the Jew first, and then to the Gentile, but Israel wouldn't believe. Even later, Peter still hoped against hope that Israel would wake up and realize the One they killed was the Promised Messiah. At that point, Peter would not address Gentiles as brethren, because they had no idea they were to have anything to do with Gentiles. The context notes Israel's ignorance in killing the Messiah, as Christ from the cross said, 'Father forgive them, for they know not what they do,' and Stephen said basically the same.