When Abram was ninety-nine years old, the Lord appeared to him and said, “I am God Almighty; walk before me faithfully and be blameless. 2 Then I will make my covenant between me and you and will greatly increase your numbers.”In the Tanakh, many people are said to see God as a physical manifestation. Or I should say many men saw him, the only woman I can find who saw God was the wife of Manoah, i.e. Samson's mother. What he looks like is usually not described, although sometimes he is said to speak to men "face to face." Moses saw God face to face a few times, and of course once as a burning bush. In Exodus, however, we are told that "There shall no man see me, and live." And the Gospel of John states repeatedly that God cannot be seen, and has never been seen. Colossians and Timothy also describe him as invisible.
3 Abram fell facedown, and God said to him, 4 “As for me, this is my covenant with you: You will be the father of many nations. 5 No longer will you be called Abram; your name will be Abraham, for I have made you a father of many nations. 6 I will make you very fruitful; I will make nations of you, and kings will come from you. 7 I will establish my covenant as an everlasting covenant between me and you and your descendants after you for the generations to come, to be your God and the God of your descendants after you. 8 The whole land of Canaan, where you now reside as a foreigner, I will give as an everlasting possession to you and your descendants after you; and I will be their God.”
As for the name change, it is not entirely clear what it's all about. Abraham may mean "Father of Many."
9 Then God said to Abraham, “As for you, you must keep my covenant, you and your descendants after you for the generations to come. 10 This is my covenant with you and your descendants after you, the covenant you are to keep: Every male among you shall be circumcised. 11 You are to undergo circumcision, and it will be the sign of the covenant between me and you. 12 For the generations to come every male among you who is eight days old must be circumcised, including those born in your household or bought with money from a foreigner—those who are not your offspring. 13 Whether born in your household or bought with your money, they must be circumcised. My covenant in your flesh is to be an everlasting covenant. 14 Any uncircumcised male, who has not been circumcised in the flesh, will be cut off from his people; he has broken my covenant.”One must wonder at the origin of this barbarous custom. One might think that it served to mark the Jews as a distinct people, but actually the custom was widespread among other peoples of the region, including the Egyptians. Note that slaves are to be circumcised.
15 God also said to Abraham, “As for Sarai your wife, you are no longer to call her Sarai; her name will be Sarah. 16 I will bless her and will surely give you a son by her. I will bless her so that she will be the mother of nations; kings of peoples will come from her.”No explanation for Sarai's name change, sorry. Remember, by the way, that the marriage is incestuous. Sarah and Abraham have the same father. Anyway, if you remember back a couple of chapters Abraham has 318 adult male slaves. He must have had some boy slaves as well, so we're talking at least maybe 400 circumcisions in a single day. Since they had presumably never done it before and didn't have any local experts, I can imagine there may have been some problems.
17 Abraham fell facedown; he laughed and said to himself, “Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?” 18 And Abraham said to God, “If only Ishmael might live under your blessing!”
19 Then God said, “Yes, but your wife Sarah will bear you a son, and you will call him Isaac. I will establish my covenant with him as an everlasting covenant for his descendants after him. 20 And as for Ishmael, I have heard you: I will surely bless him; I will make him fruitful and will greatly increase his numbers. He will be the father of twelve rulers, and I will make him into a great nation. 21 But my covenant I will establish with Isaac, whom Sarah will bear to you by this time next year.” 22 When he had finished speaking with Abraham, God went up from him.
23 On that very day Abraham took his son Ishmael and all those born in his household or bought with his money, every male in his household, and circumcised them, as God told him. 24 Abraham was ninety-nine years old when he was circumcised, 25 and his son Ishmael was thirteen; 26 Abraham and his son Ishmael were both circumcised on that very day. 27 And every male in Abraham’s household, including those born in his household or bought from a foreigner, was circumcised with him.
Anyway, while circumcision remains a requirement for Jews, the apostle Paul repudiated the practice. In fact he outright forbade it in Galatians:
Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing. For I testify again to every man that is circumcised, that he is a debtor to do the whole law. Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.Obviously Christians have chosen to ignore this passage as most of them are today circumcised shortly after birth, although only because it is a secular custom, not for religious reasons. This is starting to change, however, as some medical authorities have come to doubt that it has significant hygienic value. This is a matter of controversy.