5 And he brought him outside and said, "Look toward heaven, and number the stars, if you are able to number them." Then he said to him, "So shall your offspring be."
6 And he believed the LORD, and he counted it to him as righteousness.
Though both he and his wife were old, and his wife was well beyond the age of bearing children, God promised to Abraham that he would not only have a child, but through this child Abraham would become the father of a multitude of offspring.
God can do anything that he promises, even if there are no odds but merely an impossibility. Impossible is meaningless to God.
The amazing part here is that Abraham knew God well enough to believe him. Because Abraham honoured God by trusting him, God honoured Abraham by considering that trust as righteousness. Trust/faith/belief is the important thing to God. That trust in him will lead to actions - following God's commands about how to live. Contrast this faith with those who seek to gain approval from their actions alone. We can't earn righteousness by doing good deeds, we can only gain righteousness through faith.