A little historical back ground. By the time of Christ the nation of Israel had been under the domination of the Roman Empire for several decades. Jews had legal privileges because their ancestral laws were predated by Rome. Jews had legal privileges giving them the right to assemble, in the "Synagogue" and have common meals own property, govern them self and enforce their own discipline.
When the word "Synagogue" is mentioned in Scripture and it’s important to note this is not simply some building. The Synagogue was the only place for teaching scripture and were the members of Israel could hear the Scriptures read from their Old Testament other than (the actual Temple in Jerusalem). The leaders of Israel were also responsible for schooling children, in the Mosaic, law among other things. Jesus went the Synagogue on the Sabbath day as it was his custom. He stood up and read from the prophet Isaiah when the book was handed to him. (Luke 4:16-17)
The nation of Israel was the only non-pagan religious group in the Roman empire and the only soul keepers of the Torah תּוֹרָה, "Instruction, Teaching." The Hebrew Scriptures, referred to by Christians as the Old Testament, are called the TANAKH.
The “Gentile (“nations”), ethnikos were never keepers of the scripture. In its general framework, the Hebrew Bible is the account of God’s dealing with the Jews as his chosen people, who are collectively called Israel.
In the midst of a Roman Empire society a Christian society was founded on the Day of Pentecost in Jerusalem 30AD. Peter's sermon on the Day of Pentecost was entirely Jewish, quoting the prophets and the Psalmist David, and would have meant nothing to any Gentiles standing around, if there were any. The 3000 Jews who were saved that day would have been all Jewish. (Acts 2: 5 Now there were dwelling in Jerusalem Jews, devout men from every.
The first to believe that Jesus is the promised Messiah are Israelites— Andrew, Peter, James, John. Christianity was founded in the midst of the Roman Empire on the Day of Pentecost in Jerusalem (Acts 2) The Church was thoroughly Jewish from its earliest days (since there were no Gentiles in it from (32 to 42).
Peter's sermon on the Day of Pentecost was entirely Jewish, quoting the prophet Joe, and the Psalmist David, and would have meant nothing to any Gentiles who was not a proselyte standing around, if there were any. The 3000 people who were saved that day would have been all Jewish and some proselytes. Acts 2:5 Now there were dwelling in Jerusalem Jews, devout men from every nation under heaven.