God's covenant promise to Abraham was not to be fulfilled through just any descendant. Genesis 17:19 specifically identifies Isaac, the son to be born to Abraham and Sarah, as the one through whom the covenant would continue. This choice narrows the promised line: from Abraham the redemptive story proceeds through Isaac rather than through Ishmael or Abraham's other descendants.

Romans 9:7 cites the related statement in Genesis 21:12, "Through Isaac shall your offspring be named," while explaining God's sovereign purpose in the covenant line. The verse is not by itself a direct prediction of Jesus, but it establishes the particular family through which the Messiah would eventually come. The New Testament genealogies place Jesus within that line. Thus the promise concerning Isaac forms another stage in the progressive identification of the promised offspring: from humanity, to Shem, to Abraham, and then specifically through Isaac.