20:1-2 Abraham traveled from Mamre to Gerar, a Philistine settlement west of the Dead Sea between Kadesh and Shur, perhaps to get farther away from the devastated area where Sodom had been. Then, less than three months after God had promised that Sarah would bear Abraham a son (18:10), the patriarch gave his wife to Abimelech! Abraham had previously told pharaoh that his wife was his sister (12:12-15); later his son would try the same trick (26:7).
20:3-8 God in his mercy intervened to keep his promise regarding Sarah (18:10) from being destroyed by Abraham’s foolish act. In addition to warning Abimelech that Sarah was married, the Lord had also created a health crisis in his household, causing all the other women to become temporarily sterile (v. 18). Because Abimelech had acted with a clear conscience and clean hands—despite accepting Sarah into his harem—God would not destroy Gerar’s governmental leadership and thus undermine the nation.
Abraham is the first person to be called a prophet in the OT, though Enoch, who lived before him, is called a prophet in the NT (Jd 14). As Abraham’s intercession with God had saved Lot’s life (Gn 18:23-32; 19:29), so now his prayer for Abimelech would save his life.
20:9-18 Abimelech comes off appearing to be Abraham’s moral superior. He is penitent and also generous. A verification of your honor in v. 16 is literally “a covering of the eyes.” According to John Wenham, “the gift makes one blind to what has happened.”