Who is the son of promise in Genesis and Galatians?
Isaac is the son of promise. In Genesis, God’s promise to Abram required a son, and in Galatians the Hagar–Ishmael scenario is called after the flesh, while Isaac represents the promised son. The contrast is between human effort through the slave woman and God’s promise fulfilled through Isaac.
Les explains that in this Genesis account, Sarai speaks first and tells Abram to go in to her maid Hagar. He emphasizes that God is totally lacking in the conversation and has said nothing about having a child by the slave woman. When we get back to Galatians, the whole scenario of Hagar and Ishmael is called after the flesh, but Isaac is going to be the son of the promise of God. The difference is that Ishmael’s birth came through human action, while Isaac is tied to God’s promise.