Whose Jerusalem ?
Jesus instructed His disciples to "go only to the lost sheep of the House of Israel" and He reminded a Gentile that "Salvation is through the Jews" (John 4:22).
Subsequently, however, it became evident that salvation was designed for all mankind, beginning from Jerusalem.
We can now see that salvation started with a mere trickle of the redeemed early in the Scriptures, but that the trickle becomes a mighty river as God's plan unfolds.
Abraham believed God, and it was credited to him for righteousness; but his own personal faith attained only his own salvation, because of the personal message.
God changed the name of Abraham's grandson from Jacob to "Israel" (meaning "power with God") signifying a shift by God to a nation of people, who through faith (heart belief) in their collective deliverance by God from Egyptian slavery could attain to salvation for the nation as a whole, because of the corporate revelation.
With the coming of Jesus as Messiah, followed by His predicted rejection, crucifixion and resurrection, faith in that historical event made salvation open to all mankind.
We know that faith is not imagination or false hope. Abraham's faith worked because he had heard from God; Israel's faith worked because God had delivered Israel by the blood of the lamb; Christian faith works because "God so loved the world that He gave His son that whosoever believes in Him shall not perish but have eternal life" (John 3:16). This way of salvation by faith in God's mighty sovereign acts is the only way there is, or ever shall be, as God moves to save a remnant of humanity from otherwise certain destruction.
But note the expansion by God, of the availability of faith:
Firstly: Abraham's faith, in God's promise to him, could only save Abraham.
Secondly: Israel's faith, in God's promise to them collectively, could save all Israel.
Finally: The faith of any human in God's promise to humanity at the Cross of Calvary, opens the offer of salvation to everyone; "whosoever will may come."
An amazing expansion of the availability of salvation by faith: first to an individual, then to a nation, and finally to all mankind through Abraham, Israel and Jesus Christ.