God responds to Abraham’s questions by articulating promises to which he responds in faith.
Genesis 15 begins with a promise to Abram (God promised that Abraham would become the father of a great nation, receive a land, and bring blessing to all nations.) 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 A righteous person is one who is ethical and faithful to God's covenant. Righteousness in the Old Testament is an attitude of God; in the New Testament it is a gift of God through grace. In the New Testament righteousness is a relationship with God.... 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).