Adam Clarke on Romans 4:13

For the promise, that he should be the heir of the world - This promise intimated that he should be the medium through whom the mercy of God should be communicated to the world, to both Jews and Gentiles; and the manner in which he was justified, be the rule and manner according to which all men should expect this blessing. Abraham is here represented as having all the world given to him as his inheritance; because in him all nations of the earth are blessed: this must therefore relate to their being all interested in the Abrahamic covenant; and every person, now that the covenant is fully explained, has the privilege of claiming justification through faith, by the blood of the Lamb, in virtue of this original grant.