Although "good as dead" because he was about a hundred years old, Abraham did not waiver in his faith. Rather than focus on his age or on Sarah's deadness of womb, he grew strong in faith, being fully assured that what God had promised He was also able to perform:
And God said unto Abraham, as for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be. And I will bless her, and give thee a son also of her: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her. Then Abraham fell upon his face, and laughed, and said in his heart, shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear? (Genesis 17:15-17).Abraham, against all hope in human ability, hoped in the power and ability of God to perform what He promised. Thus Isaac was born, and a miracle nation was born.
Against all hope, Abraham in hope believed and so became the father of many nations, just as it had been said to him, "So shall your offspring be." Without weakening in his faith, he faced the fact that his body was as good as dead - since he was about a hundred years old - and that Sarah's womb was also dead. Yet he did not waiver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God, being fully persuaded that God had power to do what he had promised (Romans 4:18-21 NIV).
It is of great interest to note that many years before Isaac was born, the Lord made a covenant of blood with Abraham that the land of Canaan would be given to Abraham's seed from Isaac, and that, through Isaac, Abraham's seed would be called according to God's Word:
... for in Isaac shall thy seed be called (Gen. 21:12).The story of that blood covenant is written in Genesis 15:1-21:
After these things the word of the LORD came unto Abram in a vision, saying, "Fear not, Abram: I am thy shield, and thy exceeding great reward." And Abram said, "Lord GOD, what wilt thou give me , seeing I go childless, and the steward of my house is this Eliezer of Damascus?" And Abram said , "Behold, to me thou hast given no seed: and, lo, one born in my house is mine heir." And behold, the word of the LORD came unto him, saying, "This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir." And he brought him forth abroad, and said, "Look now toward heaven, and tell the stars, if thou be able to number them:" and he said unto him, "So shall thy seed be." And he believed in the LORD; and he counted it to him for righteousness. And he said unto him, "I am the LORD that brought thee out of Ur of the Chaldees, to give thee this land to inherit it" And he said, "Lord GOD, whereby shall I know that I shall inherit it?" And he said unto him, "Take me an heifer of three years old, and a she goat of three years old, and a ram of three years old, and a turtledove, and a young pigeon." And he took unto him all these, and divided them in the midst, and laid each piece one against another; but the birds divided he not. And when the fowls came down upon the carcasses, Abram drove them away. And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him. And he said unto Abram, "Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years; and also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance. And thou shall go to thy fathers in peace; thou shall be buried in a good old age. But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full." And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces. In the same day the LORD made a covenant with Abram, saying, "Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates: The Kenites, and the Kenizzites, and the Kadmonites, and the Hittites, and the Perrizites, and the the Rephaims, and the Amorites, and the Canaanites, and the Girgashites, and the Jebusites."During that great vision, God told Abraham that his seed would be strangers in a land that does not belong to them, and that they would be afflicted for four hundred years. What God said, He fulfilled, the seed of Abraham from Isaac spent four hundred years in Egypt before Moses got them out, and a nation was born.