Romans 4:13

For not through the law was the promise to Abraham or to his seed that he should be heir of the world, but through the righteousness of faith.

The law ... is here a reference to Moses' law; but, since that was the best ever given, it includes, by extension, every other kind of legal system.

The promise ... is the new element under consideration in this verse. Previously, in this chapter, Paul had shown that Abraham's righteousness had been reckoned unto him upon the basis of an obedient faith, prior to the giving of the law, and that even circumcision was only a seal of the righteousness that he already had; and here Paul showed that the great promise to Abraham, defined as "the promise ... that he should be the heir of the world," was given by God to Abraham long before the law of Moses (Genesis 12:1-5), thus being "not through the law." There being no Old Testament report of God's promise to Abraham in words like these, "the promise," as here stated, means all of the spiritual blessings that were to come eventually through the Messiah and his heavenly kingdom. Murray's perceptive words regarding this are:

We cannot exclude from the scope of this promise, as defined by the apostle, the most inclusive messianic purport. It is defined as the promise to Abraham that HE should he the heir of the world, but it is also a promise to his seed and, therefore, can hardly involve anything less than the worldwide dominion promised to Christ and to the spiritual seed of Abraham in him. It is a promise that receives its ultimate fulfillment in the consummated order of the new heavens and the new earth.[12]

The word "seed," as it is used of Abraham, has four distinct meanings. In the singular, it means Christ (Galatians 3:16); in the literal plural it means all the fleshly descendants of Abraham, those through Keturah and Hagar, as well as through Sarah; in the legal plural, it refers to the Jews, those who possessed the law of Moses; in the spiritual plural, it refers to baptized believers in Jesus Christ (Galatians 3:27-29); and, in the extended spiritual plural, it refers to all of the redeemed under both the old and new covenants.