In the first section (vv. 1-6) Abram seeks assurance concerning his descendant. His “complaint” arises out of faith. That is, he laments the fact that God’s promise of an heir has not yet been fulfilled precisely because he believes so strongly that God is faithful. We have further evidence that Abram’s faith is genuine and strong in the fact that God offers him assurance rather than rebuke. Abram’s faith is of the variety that cries out, “Lord I believe! Help my unbelief.”
In the second section of chapter 15 (vv. 7-21) the focus turns to God’s promise of a land that will belong to Abram and his progeny. In this section, God formalizes the everlasting covenant of grace (first hinted in Genesis 3:15) with Abram by means of a ceremony. In chapter 17 there will be a seal attached to the covenant.
As Derek Kidner writes, “God’s regular provision for such a need is ‘signs’ and ‘seals’ (Romans 4:3, 11a) to confirm the spoken word. Here the full answer is a formal covenant (vs. 18), executed in two stages. The first is in this chapter, an inauguration of a particularly vivid kind. The second stage, in chapter 17, was the giving of the covenant sign, circumcision.” [Derek Kidner, Genesis (Downers Grove: IVP, 1967) p. 124]