In chapter 15 of Genesis, we find one of the most important statements in salvation history: “Thus Abram believed YHWH, and He reckoned it to Abram as righteousness.”
A major aspect of the message of the Bible is that salvation is obtained by faith. We see this clearly in the life of Abram/Abraham.
First, God tells him to leave his home and his father’s house and to set out to a land that God will show him; from the known to the unknown. Abram trusts God and obeys. (More on this “leaving” later at some point. It is a key, repeated theme in the life of God’s people.)
Now, here in chapter 15, Abram trusts that God will fulfill His promise to make Abram a great nation even though circumstances make this seem impossible. Abram trusts God. And we see that God considers Abram “righteous” due to this trust, and not to any action of merit on Abram’s part.
In fact, Abram proves himself unworthy of such a label again and again. Even his trust/faith is shown to be weak again and again. We saw how Abram quickly lost trust when he arrived in the promised land and encountered a famine. He ran straight down to Egypt, to an earthly power for salvation then. Now, God has said that He will give Abram descendants. We are about to see that Abram will again take steps to bring this about in his own ability instead of trusting God.
It seems that Abram is not only a reminder that God requires faith alone, but that even there we are hopelessly weak. It is a good thing that the Bible does not specify that we need an unfailing trust on our own part. God provides the salvation as well as granting us the faith to accept that salvation. If it were up to us, we would be lost.