"After Abram had lived ten years in the land of Canaan, Abram's wife Sarai took Hagar the Egyptian, her maid, and gave her to her husband Abram as his wife"
New American Standard Bible. Genesis 3:16
The point is that as Sarai could not conceive ,she herself chose Hagar for her husband, so why she would do something ilegitimate as this? What some historians say is that this practice was normal and legitimate, that's why she did it in he first place!
Genesis 17:16-20 (New American Standard Bible)
16"I will bless her, and indeed I will give you a son by her Then I will bless her, and she shall be a mother of nations; kings of peoples will come from her."
17Then Abraham fell on his face and laughed, and said in his heart, "Will a child be born to a man one hundred years old? And will Sarah, who is ninety years old, bear a child?"
18And Abraham said to God, "Oh that Ishmael might live before You!"
19But God said, "No, but Sarah your wife will bear you a son, and you shall call his name Isaac; and I will establish My covenant with him for an everlasting covenant for his descendants after him.
20"As for Ishmael, I have heard you; behold, I will bless him, and will make him fruitful and will multiply him exceedingly He shall become the father of twelve princes, and I will make him a great nation.
As we see here, even in the Bible is clear that Ismael was the first son and that God blessed both Isaac and Ismael equally without disregarding the son of the 'slave'.
So clearly the christians that say that the covenant and blessings that God gave were just refering to the "legal" son and not to Ismael, should read their Bible carefully!
They should also read :
Deuteronomy 21:15-17 (New American Standard Bible)
15"If a man has two wives, the one loved and the other unloved, and both the loved and the unloved have borne him sons, if the firstborn son belongs to the unloved,
16then it shall be in the day he wills what he has to his sons, he cannot make the son of the loved the firstborn before the son of the unloved, who is the firstborn.
17"But he shall acknowledge the firstborn, the son of the unloved, by giving him a double portion of all that he has, for he is the beginning of his strength; to him belongs the right of the firstborn.
At the time, the right of the firstborn was crucial, meaning this that the first borns were the "most loved" or appreciated ,if you will; so according to this, the logic is that God would have asked Abraham to sacrifice his first born(Ismael) and not Isaac. Indeed the Bible states thcat God ask Abraham for his only son to be sacrificed (Ismael, right?) but as one more of the BIG CONTRADICTIONS it says instead:
Genesis 22:2 (New American Standard Bible)
2He said, "Take now your son, your only son, whom you love, Isaac(???), and go to the land of Moriah, and offer him there as a burnt offering on one of the mountains of which I will tell you."
May Allah guide us all.
Assalamo alikom wa rahmato Allah wa narakato.