Genesis 16 Abram has a son through a concubine

 Now Sarai, Abram's wife, bare him no children: and she had a handmaid, an Egyptian, whose name was Hagar.

2 And Sarai said unto Abram, Behold now, Jehovah hath restrained me from bearing; go in, I pray thee, unto my handmaid; it may be that I shall obtain children by her. And Abram hearkened to the voice of Sarai.

3 And Sarai, Abram's wife, took Hagar the Egyptian, her handmaid, after Abram had dwelt ten years in the land of Canaan, and gave her to Abram her husband to be his wife.

4 And he went in unto Hagar, and she conceived: and when she saw that she had conceived, her mistress was despised in her eyes.

5 And Sarai said unto Abram, My wrong be upon thee: I gave my handmaid into they bosom; and when she saw that she had conceived, I was despised in her eyes: Jehovah judge between me and thee.

6 But Abram said unto Sarai, Behold, thy maid is in thy hand; do to her that which is good in thine eyes. And Sarai dealt hardly with her, and she fled from her face.

7 And the angel of Jehovah found her by a fountain of water in the wilderness, by the fountain in the way to Shur.

8 And he said, Hagar, Sarai's handmaid, whence camest thou? and whither goest thou? And she said, I am fleeing from the face of my mistress Sarai.

9 And the angel of Jehovah said unto her, Return to thy mistress, and submit thyself under her hands.

10 And the angel of Jehovah said unto her, I will greatly multiply thy seed, that it shall not be numbered for multitude.

11 And the angel of Jehovah said unto her, Behold, thou art with child, and shalt bear a son; and thou shalt call his name Ishmael, because Jehovah hath heard thy affliction.

12 And he shall be as a wild ass among men; his hand shall be against every man, and every man's hand against him; and he shall dwell over against all his brethren.

13 And she called the name of Jehovah that spake unto her, Thou art a God that seeth: for she said, Have I even here looked after him that seeth me?

14 Wherefore the well was called Beer-lahai-roi; behold, it is between Kadesh and Bered.

15 And Hagar bare Abram a son: and Abram called the name of his son, whom Hagar bare, Ishmael.

16 And Abram was fourscore and six years old, when Hagar bare Ishmael to Abram.

Indeed, Abram had a son, but this was not the son of promise. Because of his relationship to Abram he was blessed by God. But God has future plans in store for Abram.

Genesis 15 The Abrahamic Covenant

The Abrahamic Covenant

15 After these events, the word of the Lord came to Abram in a vision:

Do not be afraid, Abram.
I am your shield;
your reward will be very great.

2 But Abram said, “Lord God, what can You give me, since I am childless and the heir of my house is Eliezer of Damascus?” 3 Abram continued, “Look, You have given me no offspring, so a slave born in my house will be my heir.”

4 Now the word of the Lord came to him: “This one will not be your heir; instead, one who comes from your own body will be your heir.” 5 He took him outside and said, “Look at the sky and count the stars, if you are able to count them.” Then He said to him, “Your offspring will be that numerous.”

6 Abram believed the Lord, and He credited it to him as righteousness.

7 He also said to him, “I am Yahweh who brought you from Ur of the Chaldeans to give you this land to possess.”

8 But he said, “Lord God, how can I know that I will possess it?”

9 He said to him, “Bring Me a three-year-old cow, a three-year-old female goat, a three-year-old ram, a turtledove, and a young pigeon.”

10 So he brought all these to Him, split them down the middle, and laid the pieces opposite each other, but he did not cut up the birds. 11 Birds of prey came down on the carcasses, but Abram drove them away. 12 As the sun was setting, a deep sleep fell on Abram, and suddenly great terror and darkness descended on him.

13 Then the Lord said to Abram, “Know this for certain: Your offspring will be foreigners in a land that does not belong to them; they will be enslaved and oppressed 400 years. 14 However, I will judge the nation they serve, and afterward they will go out with many possessions. 15 But you will go to your fathers in peace and be buried at a ripe old age. 16 In the fourth generation they will return here, for the iniquity of the Amorites has not yet reached its full measure.”

17 When the sun had set and it was dark, a smoking fire pot and a flaming torch appeared and passed between the divided animals. 18 On that day the Lord made a covenant with Abram, saying, “I give this land to your offspring,from the brook of Egypt to the Euphrates River: 19 the land of the Kenites, Kenizzites, Kadmonites, 20 Hittites, Perizzites, Rephaim, 21 Amorites, Canaanites, Girgashites, and Jebusites.”

The great nation promised Abram were the future descendants of the twelve sons of Jacob.  Jacob's name was changed to Israel and they were to mediate God's blessings to the nations. This is called the Abramhamic Covenant and is central to the Christian faith because Jesus Christ will proceed from it.