Genesis 21:12

And God said It is revealed to Abraham by night (Genesis 21:14), that compliance with Sarah's demand will be overruled to fulfil the destiny of Hagar's child. The Israelite tradition, according to the comparatively low moral standard of its time, especially in connexion with the conditions of slave concubinage and its domestic results, attributed to the voice of God a command that in our ears sounds unfeeling and cruel.

in Isaac shall thy seed be called Lit. "in Isaac shall seed be called to thee." LXX ἐν Ἰσαὰκ κληθήσεταί σοι σπέρμα, which is quoted in Romans 9:7 and Hebrews 11:18. The meaning is that in Isaac and in his descendants Abraham will have those who will be called by his name.

Isaac is to be the father of the "children of promise." He stands, therefore, in the allegory (Galatians 4:27-28), in contrast with him "that was born after the flesh" (i.e. Ishmael). Isaac stands for those "born after the spirit."