Not all Israel were considered Jews. The Jews were of the two-tribe kingdom (Judah and Benjamin) and the rest of Israel were the other ten tribes. When Jesus was at the well talking to a Samaritan woman, she hinted at what the problem was between Jews and Samaritans:
Our forefathers worshipped on this mountain, but you people say that in Jerusalem is the place where people must worship. (John 4:20)
However, Jesus affirmed something: salvation begins with the Jews (John 4:22)
Why
could he say that? Because the covenant with Abraham was not the same
as the covenant with Israel. The covenant with Abraham is that he would
be father to many nations. (Gen 17:2) The covenant with Israel is mainly
about the lineage to the Messiah would coming through him. Even before
that covenant was in place, Jacob (Israel) called his sons to him and prophesied
that the scepter would come through Judah. (Gen 49:10) Christ was from the tribe of Judah. (Matt 1:3;Luke 3:33).
Christ was sent to "the lost sheep of the house of Israel", and the apostles focused on these. But, Paul was sent to the nations, the "Gentiles".
When the blood of the covenant was poured out, that is, when Christ was executed, the new covenant went into effect. At that time, there was no consideration of the Gentiles. So, that covenant was specifically with Israel.
However, after Christ was raised, this happened:
Jesus
approached and spoke to them, saying: “All authority has been given me
in heaven and on the earth. 19 Go, therefore, and make disciples of
people of all the nations, baptizing them in the name of the
Father and of the Son and of the holy spirit, 20 teaching them to
observe all the things I have commanded you. And look! I am with you all
the days until the conclusion of the system of things.” (Matt 28:18-20)
Why did he say that? Remember, Christ was sent
ONLY to the lost sheep of the house of Israel. (Matt 15:24) He did not
attempt to preach to any Gentiles. At John 10:16, though, he said he had
"other sheep" that he must bring in. But then was not the time. It was
only after his resurrection that he extended the grace of God to the
nations. Why? Because he wanted to. Matt 28:18: "All authority has been
given me...Go, therefore..."
Other passages to consider: Gal 3:26-29; Rom 1:16-18; Eph 2:11-22; all of Romans 11