27 For it is written: “Rejoice, O barren, You who do not bear! Break forth and shout, You who are not in labor! For the desolate has many more children Than she who has a husband.” 28 Now we, brethren, as Isaac was, are children of promise.
Paul references Isaiah 54:10 to remind us that the inheritance we receive as children of God is by faith and greater than any inheritance we might receive through works.
Just as Abraham created an Ishmael through striving through works to get what had been promised him by an act of grace, we too can fall into the trap of striving to receive what has been promised by our Heavenly Father.
Isaiah 54 speaks of an eternal covenant of peace.
Our entry into that covenant is to receive by faith the sacrifice Jesus made for us to be restored to relationship with our Heavenly Father.