Sarai (or Sarah as she would become known) is a complex and ambiguous character. We first meet her in chapter 12 of Genesis when her husband Abram (later to become Abraham) is summoned by God to leave his home in the land of Haran and set out for the land of Canaan; a land which God promises to give to Abram's progeny (Genesis 12:7).
Abram almost immediately places this promise in jeopardy. In an anticipation of the Joseph story later in Genesis, Abram is forced by famine in the Promised Land to move on to Egypt. Once there, aware that his wife Sarai is very beautiful and worried by the attention she has drawn from the Egyptian men, Abram lies to Pharoah telling him that Sarai is his sister. As a result Pharoah enthusiastically takes Sarai into his hareem and 'treats Abram well' (12:16). For the moment, then, God's plan seems to be thwarted. Sarai cannot possibly concieve Abram's child and indeed is vulnerable to another man. Abram has chosen, at least for the moment, comfort in Egypt and seperation from his wife over God's promise. We are not told how Sarai feels about this. Yet God protects Sarai, he does not allow human frailty to undo his work. An understandably indignant Pharoah dismisses both Abram and his wife after his household is punished by God and Abram's deception is revealed to him, God's plan is not threatened by human sinfulness.
Sarai and Abram return to the Negeb and settle at Hebron (Genesis 13:18). In chapter 15 God renews his promise that Abram's descendents will inherit the land 'from the River of Egypt to the Great River' (15:18). Yet there is a problem: Sarai has no children and she is ageing. She comes up with a clever solution: she offers her husband her maid servant Hagar that she might 'get children through her' (16:2). Hagar does indeed concieve, but instead of handing over the child to Sarai a rivalry emerges between the two women. Sarai treats Hagar cruelly and eventually Hagar and her son Ishmael are driven away (16:6). God's purpose in establishing the chosen people can not be overcome by human sinfulness, neither can it be earned or built by human hands: Israel owes its existence to the gratuitous generosity of God.
In Chapter 17 the promise is once again renewed, God makes a covenant with Abram and gives him a new name: Abraham, 'for he will be father of many nations' (17:5). Sarai too is renamed Sarah, for God will bless her and 'she will become nations, Kings will issue from her' (17:16). In chapter 18 Abraham is visited by three mysterious strangers. He welcomes them, and they tell him that the now elderly Sarah will now concieve. Sarah, though she denies it later, laughs in disbelief when she hears (18:32). Yet these messengers from God speak the truth: what is humanly impossible is possible for God. Even though Abraham once more risks everything by allowing Sarah to be taken into the hareem of a foreign King in chapter 20, God overcomes all obstacles and blesses Sarah with a son, Isaac, in chapter 21. In her joy Sarah is able to mock herself saying: 'God has given me cause to laugh! All who hear about this will laugh with me!' (21:6).
Isaac of course would go on to be the Father of Jacob who will be renamed Israel and be father to the 12 tribes. Sarah is therefore is in a sense a quasi-Eve figure, the mother of the chosen people, the mother of the nation that will give birth to Christ. Despite the complications, difficulties, and disapointments of her life she is used by God to further His work of salvation. Sarah dies in chapter 23 and is the first to be buried in the Promised Land in the tomb of the Patriarchs bought by Abraham: a momument to Israel's claim on the land that God had promised them, a monument to God's fidelity to his word.