The basis of three fervently believed and closely-held religions, Judaism, Christianity, and Islam is circumspect because the poorly written Genesis story is bullshit. Although the submission would not pass any modern-day writer's critique it does show us a very dysfunctional blended family.
The Lord had said to Abraham, “Go from your country, your people and your father’s household to the land I will show you.
“I will make you into a great nation, and I will bless you; I will make your name great, and you will be a blessing.
I will bless those who bless you, and whoever curses you I will curse; and all peoples on earth will be blessed through you."
So Abraham went, as the Lord had told him; and Lot went with him. Abraham was seventy-five years old when he set out from Harran. He took his wife Sarah, his nephew Lot, all the possessions they had accumulated and the people they had acquired in Harran, and they set out for the land of Canaan, and they arrived there.
The Lord appeared to Abraham and said, “To your offspring I will give this land.” So he built an altar there to the Lord, who had appeared to him.
Wait a minute here... How can God promise a 75 year old fellow his offspring will be blessed. Abraham has no sons or daughters, and he is old!
Genesis 15 tells of the covenant God made with Abraham:
"When the sun had set and darkness had fallen, a smoking firepot with a blazing torch appeared and passed between the pieces. On that day the Lord made a covenant with Abraham and said, “To your descendants I give this land, from the Wadi[e] of Egypt to the great river, the Euphrates— the land of the Kenites, Kenizzites, Kadmonites, Hittites, Perizzites, Rephaites, Amorites, Canaanites, Girgashites and Jebusites.”
Problems fulfilling the covenant immediately emerge. Abraham had no descendant. He was seventy-five years old and he had no offspring. It would be hard to have kids. His wife, Sarah was of similar age, but was also impressed with the Lord's promise. That's why she suggested her husband sleep with Hagar, their Egyptian slave.
Genesis 16: Now Sarah, Abraham’s wife, had borne him no children. But she had an Egyptian slave named Hagar; so she said to Abraham, “The Lord has kept me from having children. Go, sleep with my slave; perhaps I can build a family through her.”
Deciding he needed offspring to uphold his part of God's great promise, the grandpa-aged Abraham slept with Hagar. It took ten years to make Hagar pregnant, but Abraham finally pulled it off.
So Hagar bore Abraham a son, and Abraham gave the name Ishmael to the son she had borne. Abraham was eighty-six years old when Hagar bore him Ishmael.
But then Sarah, feeling neglected those ten years became bitchy towards Hagar. She blames Abraham.
Then Sarah said to Abraham, “You are responsible for the wrong I am suffering. I put my slave in your arms, and now that she knows she is pregnant, she despises me. May the Lord judge between you and me.”
Tiring of being blamed for the discord Abraham throws the problem back to his wife.
“Your slave is in your hands,” Abraham said. “Do with her whatever you think best.”
Unable to be near the bitchy Sarah, Hagar leaves.
Then Sarah mistreated Hagar; so she fled from her.
Evidently this fleeing was contrary to God's wise plan for humanity's future. He persuades Hagar to return by making her a promise.
The angel of the Lord found Hagar near a spring in the desert; it was the spring that is beside the road to Shur. And he said, “Hagar, slave of Sarah, where have you come from, and where are you going?”
“I’m running away from my mistress Sarah,” Hagar answered.
Then the angel of the Lord told her, “Go back to your mistress and submit to her.” The angel added, “I will increase your descendants so much that they will be too numerous to count.”
Soon after making Hagar the promise that her son would have a shit load of offspring, Almighty God decides he doesn't like the boy .
"He (Ishmael) will be a wild donkey of a man, His hand will be against everyone, And everyone's hand will be against him; And he will live in the presence of his brethren."(Genesis 16).
Being unsatisfied with Ishmael, God devised a back-up plan. He made a new covenant with Abraham promising the gray haired dude he could have a son with Sarah if he had the one on one with his dried and wrinkled wife.
The Lord appeared to Abraham near the great trees of Mamre while he was sitting at the entrance to his tent in the heat of the day. Abraham looked up and saw three men standing nearby. When he saw them, he hurried from the entrance of his tent to meet them and bowed low to the ground.
He said, “If I have found favor in your eyes, my lord, do not pass your servant by. Let a little water be brought, and then you may all wash your feet and rest under this tree. Let me get you something to eat, so you can be refreshed and then go on your way—now that you have come to your servant.”
“Very well,” they answered, “do as you say.”
So Abraham hurried into the tent to Sarah. “Quick,” he said, “get three seahs of the finest flour and knead it and bake some bread.”
Then he ran to the herd and selected a choice, tender calf and gave it to a servant, who hurried to prepare it. He then brought some curds and milk and the calf that had been prepared, and set these before them. While they ate, he stood near them under a tree.
“Where is your wife Sarah?” they asked him.
“There, in the tent,” he said.
Then one of them said, “I will surely return to you about this time next year, and Sarah your wife will have a son.”
Well maybe Sarah and Abraham didn't need a one on one. This may have been an immaculate conception, maybe one of the angelic guests impregnated Sarah. Either way, the credit goes to the old man; according to the Holy Book Abraham fathered both Ishmael and Isaac.
Genesis 21: Now the Lord was gracious to Sarah as he had said, and the Lord did for Sarah what he had promised. Sarah became pregnant and bore a son to Abraham in his old age, at the very time God had promised him. Abraham gave the name Isaac to the son Sarah bore him. Abraham was a hundred years old when his son Isaac was born to him.
God's divine plan fell apart when the boys didn't get along. Almighty God sided with Isaac. And so God casts Ishmael out while promising that his descendants would prosper. WTF?
On the day of feasting during which Abraham celebrated the weaning of Isaac, Ishmael was mocking Isaac and Sarah asked Abraham to expel Ishmael and his mother, saying: "Get rid of that slave woman and her son, for that slave woman's son will never share in the inheritance with my son Isaac." Her demand was painful for Abraham, who loved Ishmael. Abraham agreed only after God told him that "in Isaac your seed shall be called", and that God would "make a nation of the son of the bondwoman" Ishmael, since he was a descendant of Abraham (Genesis 21:11–13), God having previously told Abraham "I will establish My covenant with [Isaac]", while also making promises concerning the Ishmaelite nation (Genesis 17:18–21).
Pissed off at his father's doctrine, Ishmael separated from the family. He prospered, and his offspring developed the Islamic fate.
Ishmael's offspring founded Islam while Isaac's descendants drifted into Judaism. Christianity developed later from Isaac's descendants.
I think God Almighty didn't have a clue what he was doing. Sort of like tRump acting presidential.