Bible Notes Online - Genesis 20 - ESV
Commentary

v1: Abraham journeys south, as in 12.9-10, but not so far as Egypt.

v2: Another lie, see 12.11-15, and another batch of problems.

v3-7: Demonstrating His sovereignty and grace, God speaks in a dream to Abimelech. Some dreams need explanation, but this one is clear and, presumably, verbal. Dreams are more common in Genesis, as perhaps God's main way of speaking. Later He gives priests and prophets, who deliver God's word.

The Lord preserves Abraham, reproving kings for his sake, Ps 105.12-15. It seems that the Lord is more strict with Abimelech than with Abraham; but the truth is that Abraham is God's man, the prophet, whose prayers bring blessing to others.

v8-10: Abimelech reproves Abraham; as in v4-5; here is a righteous nation, people of integrity, a high standard in the lives of unbelievers; but this is not common, see Rom 2.14-15.

v11-13: Abraham's fear of the people of Gerar; he has judged that they are not a godly people, and that his life would be at risk; in this respect he has misjudged them. His fear lead to him speaking a lie about his relationship with Sarah.

Again, this is a lesson for him, as he seeks to walk with God.

v14-15: The Lord preserves Abraham, and prospers him. Whatever happens to him, wherever he goes, he remains God's man, the one to inherit the promises of God.

v16: The offence against Sarah; Abimelech regretted his actions, and pays Abraham money as an atonement.

v17-18: It seems that Abraham and Sarah were with Abimelech for several months, during which the Lord closed the wombs of the women of Abimelech's family. Isaac is born during this time also; 21.22 finds Abraham still with Abimelech.