Genesis 20:1 And Abraham journeyed from there to the South, and dwelt between Kadesh and Shur, and stayed in Gerar.
We are not told this occasion of this journey, "It is very likely that this holy man was so deeply affected with the melancholy prospect of the ruined cities, and not knowing what was become of his nephew Lot and his family, that he could no longer bear to dwell within sight of the place.” (Adam Clarke) For what ever reason, he goes to live in the land of the Philistines
2 Now Abraham said of Sarah his wife, “She is my sister.” And Abimelech king of Gerar sent and took Sarah.
Compare to a similar experience in Genesis 12:13. Here, we are told of no famine; "the difficulty in which he finds himself is occasioned by Abraham himself. On the very eve of the fulfilment of the long-hoped-for promise, Abraham, largely through his own sins, imperils the precious hope. So once again, as so often in Genesis, the sovereign mercy of God is made to stand forth as supreme, that no flesh may glory before God.” (H C Leupold)
3 But God came to Abimelech in a dream by night, and said to him, “Indeed you are a dead man because of the woman whom you have taken, for she is a man’s wife.”4 But Abimelech had not come near her; and he said, “Lord, will You slay a righteous nation also? 5 Did he not say to me, ‘She is my sister’? And she, even she herself said, ‘He is my brother.’ In the integrity of my heart and innocence of my hands I have done this.” 6 And God said to him in a dream, “Yes, I know that you did this in the integrity of your heart. For I also withheld you from sinning against Me; therefore I did not let you touch her. 7 Now therefore, restore the man’s wife; for he is a prophet, and he will pray for you and you shall live. But if you do not restore her, know that you shall surely die, you and all who are yours.”
"Thus we find that persons who were not of the family of Abraham had the knowledge of the true God. Indeed, all the Gerarites are termed צדיק גוי goi tsaddik, a righteous nation, Genesis 20:4.” (Adam Clarke) “Had Abraham entertained a just opinion of the prince and people of Gerar, or taken pains to become acquainted with them, before he listened to the secret whispers of jealousy, he would have shunned so dangerous an artifice as to disguise his relation to his wife, and would have prevented the mischiefs which ensued, and the still greater mischiefs which threatened his own family and the house of Abimelech. It was a special Divine interposition which averted consequences of the most serious nature.” (The Biblical Illustrator)
8 So Abimelech rose early in the morning, called all his servants, and told all these things in their hearing; and the men were very much afraid. 9 And Abimelech called Abraham and said to him, “What have you done to us? How have I offended you, that you have brought on me and on my kingdom a great sin? You have done deeds to me that ought not to be done.” 10 Then Abimelech said to Abraham, “What did you have in view, that you have done this thing?”
"Abimelech gives evidence of prompt obedience. It required a measure of humility to tell these things to his servants; but the servants had also been involved in the misdeed, even as they would have shared the punishment if the evil had not been adjusted. The servants have their master’s attitude of respect and reverence for God, which here at the news of the danger they had incurred made them to be much afraid.” (H C Leupold)
11 And Abraham said, “Because I thought, surely the fear of God is not in this place; and they will kill me on account of my wife. 12 But indeed she is truly my sister. She is the daughter of my father, but not the daughter of my mother; and she became my wife. 13 And it came to pass, when God caused me to wander from my father’s house, that I said to her, ‘This is your kindness that you should do for me: in every place, wherever we go, say of me, “He is my brother.”’”
Abraham was to be the father of the nation created to propagate the covenant. (Exodus 19:6) But here we find another obedient nation-- a picture of the gentile nation. (1 Peter 2:9-10) Yahweh "God must stand as sole guarantor of the covenant. Abraham's failure is a result of a wrong perception of the situation (v. 11), fear (v. 11b), the casuistry of a half-truth (v. 12), and expediency (v. 13). And, as his righteous action led to the saving of another in the previous chapter, so here his faithlessness jeopardizes the life of another (v. 3).” ( Asbury Bible Commentary)
14 Then Abimelech took sheep, oxen, and male and female servants, and gave them to Abraham; and he restored Sarah his wife to him. 15 And Abimelech said, “See, my land is before you; dwell where it pleases you.”
Abimelech regards the Words of Yahweh— "he is a prophet." And thus, the gifts of sheep, oxen and servants were tokens of the high esteem that he had for Abraham and his wife for all to see. And he restored Sarah, his wife to him. Sojourn where you like for as long as you like. There is no strife between us. We are brethren.
16 And unto Sarah he said, Behold, I have given thy brother a thousand pieces of silver: behold, he is to thee a covering of the eyes, unto all that are with thee, and with all other: thus she was reproved. [KJV]
"Abimelech is not said in v. 14 to have given Abraham any silver; but here he refers to his gift as something that has been made. Consequently, the thousand shekels of silver may be the value of the gift just confessed and not an additional gift. Considering that thirty pieces was the normal price of a slave, it would seem that he considered what he had bestowed in v. 14 a princely gift..." (H C Leupold) She was bought at a price by her spiritual relative and given back to her husband.
a covering of the eyes.... This whole incident is a blot on her record and an embarrassment to her. Her supposed brother is a covering to her eyes.. "in presence of thy domestics and of all with whom thou mayest yet mingle, … Abraham will be thy best defense." (Pulpit Commentary) Stick to him.
"Thus she was rebuked. “The ancient Hebrew word for reproved is ‘yakach.’ It has the idea of 'set right,' so it is debatable as if Sarah was 'set right' by Abimelech’s rebuke, or if she was 'found to be right' because of her humble submission in this occasion. In a sense, both are true.” (David Guzik) "In all respects thou art justified.”
17 So Abraham prayed to God; and God healed Abimelech, his wife, and his female servants. Then they bore children;18 for the Lord had closed up all the wombs of the house of Abimelech because of Sarah, Abraham’s wife.
Jesus said: "And other sheep I have which are not of this fold.” (John 10:16) The contents of the covenant are given to Abraham, but extends to all. Likewise, the mission of Messiah was first to the Jew, then to the gentiles. Yahweh’s blessing of spiritual fruitfulness extend to all of His people.