And when they were come, and had gathered the church together, they rehearsed all that God had done with them, and how he had opened the door of faith unto the Gentiles.
Note 3 at Ac 14:27: Jesus administered miraculous cures to some Gentiles during His earthly ministry (the centurion and his servant, Mt 8:5-13 and Lu 7:1-10, and the Syrophenician woman and her daughter, Mt 15:21-28 and Mr 7:24-30), but it was clear that His earthly ministry was to the people of Israel (Mt 15:24). However, after His resurrection and ascension, His followers were to share the Gospel with everyone, Jews and Gentiles (see note 1 at Joh 12:23).
Philip shared the Gospel with the Ethiopian eunuch, who was either a Gentile or a proselyte to Judaism (Ac 8:26-39). Peter's miraculous experience with Cornelius (Ac 10) had been debated by the leaders of the church (Ac 11:1-18), who admitted that the Lord had "granted repentance unto life" (Ac 11:18) to the Gentiles. This caused some disciples, who had been scattered from Jerusalem (Ac 11:19), to share the Gospel with individual Gentiles (Ac 11:20-21), but no one had ever actively sought to evangelize the Gentiles as Paul and Barnabas did.
This was a dramatic new development in the preaching of the Gospel, and it caused such uproar among the Jewish believers that a special council of the Jerusalem church elders was convened (Ac 15:6). Paul and Barnabas (Ac 15:2, 4, and 12), as well as Peter (Ac 15:7-11), gave testimony of how the Lord had granted salvation to the Gentiles through faith alone, without their conversion to Judaism, and the elders gave their blessing to Paul to be the apostle to the Gentiles (Ro 11:13 and Ga 1:15-16).
Millions upon millions of Gentiles have professed faith in the Lord Jesus Christ over the centuries, and the Gentiles have actually been the ones that the Lord chose to preserve Christianity after the Jews as a whole rejected it. All of this came as a result of Paul's first missionary journey.