When Abraham was 99 years old an angel representing the Almighty visited Abram and Sarai and gave the covenant of circumcision and told them that their names were to be changed. Abram's name was changed to Abraham, and Sarai's to Sarah. Abram meant "lofty father" and Abraham was "father of a multitude" - emblematic of the promise of Genesis 12verses3; i.e. of all nations in the world. The divine name of Yahweh is 4 letters in the Hebrew YHWH (called the Tetragrammaton). So, both Abraham and Sarah become "heirs together in the grace of life" (1 Peter 2verses7); by the addition of the 5th letter in the Hebrew alphabet, signifying "grace". It was when human worth and generative capability was repudiated that God could now work through Abraham and Sarah to produce the "promised seed”, the heir of the promises.
The chapter concludes with the circumcision of Abraham and every male in his household. Abraham was, as we said, 99 and Ishmael 13 (the age at which Islamic boys are circumcised). Chapter 18 says Abraham and Sarah were visited by 3 men, who are in fact angels - unbeknown to Abraham and Sarah (prompting Paul's comments in Hebrews 13verses2). Eastern hospitality was shown and the angels prophesy Isaac's birth which means "laughter". He was so named because Sarah initially refused belief; but, later in faith tells us that she conceived when she believed - Hebrews 11verses11. After this Abraham walked with the angels towards the place of impending judgment - Sodom. They tell Abraham of their intentions, because of Abraham's faithfulness - slowly read aloud the words of verses 17-19; and pause and ponder. How wonderful that the LORD so knows each of us. But has he foreseen diligence and faithfulness in our dealings with our families? Abraham enters into negotiations with the name bearing angel to spare Sodom, where his nephew Lot and his family lived. Abraham finally stopped pleading for the city to be spared should there be 10 righteous people in the city. He must have been confident of this given the vast numbers of Lot's "ecclesia" in Genesis 13.