At the time when Paul wrote Galatians, Hagar and Ishmael (the Old Covenant and it’s children-Israel according to the flesh) were not yet “cast out” (excluded from the inheritance in Abraham), but were about to be. Those in Christ (including the faithful remnant of Israel according to the flesh) were about to receive the Abrahamic inheritance through the establishment of the New Covenant in AD70, in fulfillment of the promise made to Abraham.
The Institute Of Fulfilled Eschatology