Genesis 15 begins with a promise to Abram (Abraham) and his subsequent question about whether he will continue childless (15:1-3). God responds to him by making promises that include not only an heir but innumerable descendants (15:4-5).
Unlike his response to the divine promise of 15:1, Abraham believes--that is, he trusts in the one to whom his faith clings (15:6; see also Romans 4; Galatians 3). Abraham fixes his heart on God, rests back in the arms of the promise-giver. Abraham believes God's promise without having any concrete evidence that God's promise will come to pass (see also Hebrews 11:1, 8-12). Abraham's faith has been enabled by God's promises. God's word makes Abraham's faith possible, indeed creates his faith. Abraham's faith arises not from within him or by his own resources.
It is important to note that Abraham's faith has been enabled because God particularizes the promise by addressing the specific situation opened up by Abraham's question (he did not have this response to God's promise in 15:1). It is God's promise to very particular needs that generates Abraham's faith.
Abraham's faith was reckoned to him as righteousness (15:6). In response to Abraham's faith, God formally declares that Abraham is righteous. Abraham's righteousness refers to Abraham's right relationship with God, which occurs by virtue of God's declaration to him in view of his faith. In later texts, righteousness will often mean doing justice to this relationship in which one already stands (for example, 18:23-26).
15After these things the word of the Lord came to Abram in a vision, Do not be afraid, Abram, I am your shield; your reward shall be very great. 2But Abram said, O Lord God, what will you give me, for I continue childless, and the heir of my house is Eliezer of Damascus?* 3And Abram said, You have given me no offspring, and so a slave born in my house is to be my heir. 4But the word of the Lord came to him, This man shall not be your heir; no one but your very own issue shall be your heir. 5He brought him outside and said, Look towards heaven and count the stars, if you are able to count them. Then he said to him, So shall your descendants be. 6And he believed the Lord; and the Lord * reckoned it to him as righteousness.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org