Genesis 22:15-18 - And the angel of the Lord called unto Abraham out of Heaven the second time, 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; that in blessing I will bless thee, and in muliplying I will multiply thy seed as the stars of heaven, and as the sand that is upon the sea shore; and thy seed shall posess the gate of his enemies; and in thy seed shall all nations be blessed; because thou hast obeyed my voice".
Galations 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.
In Genesis, God made a promise to Abraham that a future descendant will be a blessing to many and that Abraham will have many future generations that will be impossible to number because there will be so many. God made this promise because of Abraham's faithfulness in following God.