Gen 21:2-3, Gen 22:16-18, Gal 3:16, Rom 9:7-8, Gal 4:21-23
God promised to send Christ through the descendants of Abraham's son Isaac. (Genesis 17:19,21, Gen 21:2-3, Romans 9:7-8, Gal 4:21-23)
Genesis 17:19 And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him. 21 But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year.
Gen 21:2 For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him. 3 And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac.
Romans 9:7 Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. 8 That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.
Gal 4:21 Tell me, ye that desire to be under the law, do ye not hear the law? 22 For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. 23 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.
All people will be blessed through Christ who will come as Abraham's descendant. (Gen 22:16-18, Gal 3:16)
Gen 22:16 And said, By myself have I sworn, saith the Lord, for because thou hast done this thing, and hast not withheld thy son, thine only son: 17 That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies; 18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.
Gal 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.