וַיֹּ֣אמֶר אֱלֹהִ֗ים אֲבָל֙ שָׂרָ֣ה אִשְׁתְּךָ֗ יֹלֶ֤דֶת לְךָ֙ בֵּ֔ן וְקָרָ֥אתָ אֶת־שְׁמ֖וֹ יִצְחָ֑ק וַהֲקִמֹתִ֨י אֶת־בְּרִיתִ֥י אִתּ֛וֹ לִבְרִ֥ית עוֹלָ֖ם לְזַרְע֥וֹ אַחֲרָֽיו׃
Lalu Allah berfirman: “Sesungguhnya, Sara, istrimu, akan melahirkan seorang anak laki-laki bagimu, dan engkau harus menamai dia Ishak. Aku akan meneguhkan perjanjian-Ku dengan dia sebagai perjanjian yang abadi bagi keturunannya sesudah dia.
Kedushat Levi
Once we appreciate this we can also understand why in parshat Lech lecha prior to Avram being commanded to circumcise himself, G’d promised him that he would father a son from Sarah. (Genesis 17,15-16) This meant that the reward for all the commandments Avraham would perform subsequently would not be offset against accumulated merits in respect of miracles G’d would perform for him. These “miracles” would be viewed as fulfillment of what G’d “owed” him, i.e. promises made but not yet fulfilled.