Jamieson, Fausset and Brown on Jeremiah 31:31

31. the days ... new covenant with ... Israel ... Judah--The new covenant is made with literal Israel and Judah, not with the spiritual Israel, that is, believers, except secondarily, and as grafted on the stock of Israel (Ro 11:16-27). For the whole subject of the thirtieth and thirty-first chapters is the restoration of the Hebrews (Jr 30:4, 7, 10, 18; 31:7, 10, 11, 23, 24, 27, 36). With the "remnant according to the election of grace" in Israel, the new covenant has already taken effect. But with regard to the whole nation, its realization is reserved for the last days, to which Paul refers this prophecy in an abridged form (Ro 11:27).