The promise made to Abraham and his offspring that he should inherit the world, did not reach him through the Law, but through the righteousness of faith.
It was by faith, when Abraham was put to the test, that he sacrificed Isaac, he was ready to sacrifice his only son, although he had received the promises
Now the Promises were made to Abraham and to his offspring; it is not said, 'and to your offsprings' in the plural, but in the singular and to your offspring — which is Christ.
Now you are the sons of the prophets and of the covenant which God made with your fathers when he said to Abraham, all families on earth shall be blessed in your offspring.
(These all died in faith without obtaining the promises; they only saw them far away and hailed them, owning they were 'strangers and exiles upon earth.'