Genesis 21:1-4

Genesis 21:1-4

[1] And the LORD  visited  Sarah  as he had said,  and the LORD  did  unto Sarah  as he had spoken.  [2] For Sarah  conceived,  and bare  Abraham  a son  in his old age,  at the set time  of which God  had spoken  [3] And Abraham  called  the name  of his son  that was born  unto him, whom Sarah  bare  to him, Isaac.  [4] And Abraham  circumcised  his son  Isaac  being eight  days  old,  as God  had commanded 

What is the context of Genesis 21:1-4?