Isn’t the Covenant in Scripture Explicitly with Israel and Not the Gentile Nations?

Questions about what gives modern Christians confidence that they’ve inherited the Jews’ covenant with God if Scripture explicitly says it was with Israel and not the Gentile nations, and why the Jews were chosen.  

• The covenant in Scripture is explicitly with Israel and not the Gentile nations, so what gives modern Christians confidence that they’ve inherited that covenant?
• Why were the Jews chosen?