It would be difficult to overestimate the importance of Genesis 15. It contains within it the principle of justification by faith and the formalizing of God’s everlasting covenant of grace. The promise God makes to Abram and ratifies through a formal ceremony is the very promise (or covenant) that Jesus Christ came to fulfill; the very promise under which Christians are saved today.
Chapter 15 describes two Divine encounters (vv. 1-6 & 7-21). Both encounters involve a dialogue between Abram and Yahweh. There are also signs pointing symbolically to God’s presence and promises. In the first encounter, God speaks to Abram by night by means of a vision concerning the promised seed. The second encounter takes place at dusk and concerns the promised land.1
1 Bruce Waltke, Genesis: A New Commentary (Grand Rapids: Zondervan, 2001) p. 238