The separation between Jews and Gentiles is well known. That separation was deemed necessary by God in order that a pure line from Abraham to Christ might be maintained and to help the Jew retain his faith in the one true God.
This separation was not to be permanent, however. In His promise to Abraham of his coming Seed, the Lord said that in that Seed “all the nations of the earth would be blessed”. The prophets also were unanimous in declaring that all nations would be blessed in the coming Christ. Finally, when Jesus, the promised Messiah, did come His charge to His apostles who would execute His commission to them after He returned to heaven was: “Go therefore and teach all nations” (Matthew 28:18), and “Go into all the world and preach the gospel to every creature” (Mark 16:15). In Peter’s first sermon on Pentecost he reminded the multitude that the gospel was designed and intended for everyone for in Joel’s promise of the coming Spirit. He said, “And whosoever shall call upon the name of the Lord shall be saved” (Joel 2:30).
According to Adam Clarke, ten years likely passed between Pentecost (Acts 2) to Acts 10 where the first actual Gentile was converted. Whether that is an accurate number we cannot vouch, but we know it had been several years. The gospel had spread from Jerusalem to Judaea, then to Samaria, and was now poised to experience a rapid spread throughout the whole world.
God was preparing to send His apostle Saul to the Gentiles. Saul has been converted, spent time in Damascus, three years in Arabia, back to Damascus, then to Jerusalem for fifteen days, and finally, when the Jews threatened his life, those brethren sent him to Tarsus (Acts 9:30). Yet, while it will be Paul who for the next thirty years will dominate Luke’s account of the gospel’s spread (his efforts in carrying the gospel to the Gentiles in Acts 13-20), it would not be Saul who was given the right to preach the gospel to the first Gentile. That was given to Peter. When Jesus had asked His disciples whom men said He was, and then when they answered, asked whom they said He was, Peter immediately responded, “Thou art the Christ, the Son of the living God”. Jesus then responded to Peter, “Blessed art thou Simon Bar Jonas. Flesh and blood hath not revealed this unto thee, but my father who is in heaven. And I also say unto thee that thou art Peter and upon this rock I will build my church; and the gates of Hades shall not prevail against it. I will give unto thee the keys of the kingdom of heaven and whatsoever thou shall bind on earth shall be bound in heaven and whatsoever thou shalt loose on earth shall be loosed in heaven” (Matthew 16:17-19). Peter first used the “keys to the kingdom of heaven” when he preached to the multitude of Jews on Pentecost (Acts 2:37-38). He used those keys a second time when he preached the gospel to Gentiles in Caesarea (Acts 10:34-43).
Cornelius was a Gentile who had come (through what means we do not know) to believe in the God of Israel — the One, Eternal God. The Holy Spirit described Cornelius thusly: “a devout man, and one that feared God with all his house, who gave much alms to the people and prayed to God always” (Acts 10:2). Cornelius kept the Jewish tradition of praying at a certain time. When the angel God sent to speak to him, Cornelius was keeping the “ninth hour of prayer”. He was what the Jews called a “proselyte of the gate”. He abstained from idolatry, kept the law of God, but he did not accept circumcision. While he was respected by the Jews, he was not one with whom they would personally associate nor into whose house they would enter. When Peter did enter Cornelius’s house, his brethren in Jerusalem were upset with him and said, “Thou wentest into men uncircumcised and didst eat with them” (Acts 11:3). Peter’s own brethren were talking about Cornelius!
Aside from the fact that Cornelis was uncircumcised and the Jews were, he was no different from the devout Jews of the apostles’ day who believed in God but did not believe in Jesus. He, like they, needed to hear and obey the gospel so that he could be saved. And now, in Acts 10, he will given that opportunity!