Isaac... thy seed. Here ser'a is in the sing sense, because of the word "Isaac", and because of the sing verb "it shall be called. " Zer'a is. collective noun (like Eng. "sheep"), but the context must determine whether it is sing, or plural. It is to this verse Galatians 1:3; Galatians 1:16 refers; not to Genesis 12:7, where it is indefinite; or Genesis 17:7 where the verb and pronouns show it is plural. See note there, and on Galatians 1:3; Galatians 1:16. and compare Romans 9:7; Hebrews 11:18. "Thy seed" is therefore "Christ". The difference of the 30 years comes in here: 430 to the Exodus (Gen 12:40) from Genesis 12:4, when Abraham was 75: 25 thence to Isaac's birth: and now,. to his recognition as the seed. 30 years.
Related Commentaries of Genesis 21:12
Genesis 21:12
12 And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy bondwoman; in all that Sarah hath said unto thee, hearken unto her voice; for in Isaac shall thy seed be called.