The passage begins, "After these things..." I looked back to see what these things had been. They include the rescue by Abraham (He's still called Abram at this point) of his nephew Lot who had been captured by an army in their sack of Sodom. The kings in the area, including Melchizedek who was also a priest, had blessed Abraham in recognition of his defeat of their shared enemies. The king of Sodom had tried to reward Abraham but he refused taking anything that would make him seem a beneficiary of those powers (14).
Abraham is there because the Lord had told him to move to this place (12:1-3). When they had arrived in Canaan, the Lord had informed Abraham that although the land currently belonged to the Canaanites, in the future it would belong to Abraham's descendants (12:4-7). Before there were any descendants, though, Abraham allowed his wife, Sarah (whose name has not yet been changed from Sarai), to be taken into the Pharaoh's harem. The Lord intervened, and Abraham got his wife back (12:10-20).
But still no children by Sarah.
In a vision, the word of the Lord came to Abraham. First, the Lord reassures Abraham then tells him he will be rewarded very well. Protection right now and, later, rewards to come.
Abraham does not respond immediately with awe and gratitude. Rather, he reminds the Lord that the previous promise of descendants had not even begun to be fulfilled.
Can Abraham believe the shield part if he doesn't believe the rest of the promise? Remember, he has moved his family a large distance because he had believed what the Lord had told him.
When Abraham doubts, he expresses those doubts openly and directly.
The word of the Lord comes to Abraham. "Look at the sky and count the stars. There are too many to count. That's how many descendants you are going to have."
We are told that Abraham believed the Lord.
Side points: The commentary to verse 6 in the New Interpreter's Study Bible points out that the word translated as "believe" also means "trust" and that the New Testament authors interpreted this verse in contrasting ways:
The apostle Paul, in his explanation of God's inclusion of the Gentiles into the kingdom of God, later interpreted this verse to mean that faith apart from the works of the Law, is the ultimate basis for salvation. By contrast, the Letter of James interprets this verse to mean that works must accompany faith.