"After these things the word of the LORD came to Abram in a vision, saying, 'Do not be afraid, Abram. I am your shield, your exceedingly great reward.' But Abram said, 'LORD God, what will You give me, seeing I go childless, and the heir of my house is Eliezer of Damascus?' Then Abram said, 'Look, You have given me no offspring; indeed one born in my house is my heir!' And behold, the word of the LORD came to him, saying, 'This one shall not be your heir, but one who will come from your own body shall be your heir.' Then He brought him outside and said, 'Look now toward heaven, and count the stars if you are able to number them.' And He said to him, 'So shall your descendants be.' And he believed in the LORD, and He accounted it to him for righteousness" (Gen. 15:1-6).
It appears that the vision Abram experiences here lasts the entire chapter. He dialogues with God and the Almighty formally confirms the covenant (specifically the nation and land promises) He had made earlier. God encourages Abram not to fear for the LORD would be his protection and would reward him greatly. This leads Abram to note that God had not yet rewarded him with any offspring. Likely dying with no heir is what Abram feared the most! God assures Abram that his heir would come from his own body; he would have a son when the time was right. Abram is told that his descendants would be innumerable like the stars. The text notes that Abram took God at His word - "He believed in the LORD." The old man Abram who was childless at that time believed what God said about His descendants. Since there was nothing more for him to do at this time except believe, God "accounted it to him for righteousness."
"Then He said to him, 'I am the LORD, who brought you out of Ur of the Chaldeans, to give you this land to inherit it.' And he said, 'LORD God, how shall I know that I will inherit it?' So He said to him, 'Bring Me a three-year-old heifer, a three-year-old female goat, a three-year-old ram, a turtledove, and a young pigeon.' Then he brought all these to Him and cut them in two, down the middle, and placed each piece opposite the other; but he did not cut the birds in two. And when the vultures came down on the carcasses, Abram drove them away" (Gen. 15:7-11).
God here reiterates the land promise. Abram inquires as to how he can truly know if he will inherit it or not. God instructs him to gather five specific animals. Abram does so (while still in the vision) and cuts the heifer, goat, and ram in half. He lays each half opposite the other, with room to pass between the two pieces. Abram, as he waits for God, prevents the vultures from having a feast.
"Now when the sun was going down, a deep sleep fell upon Abram; and behold, horror and great darkness fell upon him. Then He said to Abram: 'Know certainly that your descendants will be strangers in a land that is not theirs, and will serve them, and they will afflict them four hundred years. And also the nation whom they serve I will judge; afterward they shall come out with great possessions. Now as for you, you shall go to your fathers in peace; you shall be buried at a good old age. But in the fourth generation they shall return here, for the iniquity of the Amorites is not yet complete.' And it came to pass, when the sun went down and it was dark, that behold, there appeared a smoking oven and a burning torch that passed between those pieces. On the same day the LORD made a covenant with Abram, saying: 'To your descendants I have given this land, from the river of Egypt to the great river, the River Euphrates" (Gen. 15:12-18).
Abram, who is still experiencing a heavenly vision, here falls into a "deep sleep." He has a nightmare, so to speak, sent from God! Not only does God tell him that he would have many descendants, but now He informs Abram that these descendants would be slaves in a foreign land for several centuries! However, when the time was right (i.e., the fourth generation), their captors (the Egyptians) would be divinely judged and Abram's people would be freed (and they would leave Egypt with many possessions). They would come to the Promised Land (Canaan), for then it would be time for the Amorites to be driven out. God would not arbitrarily take the land away from its current inhabitants. Their sins were not yet "overflowing" at this point, though they would be after four more generations and God would destroy them and give the land to Abram's descendants. Today, where we might shake hands or sign our name to confirm an agreement or covenant, back then it was a tradition to cut an animal in half (representing the two sides of the agreement) and then pass between the pieces (signifying unity). God passed between the carcasses, not in the form of a man, but in the form of a "smoking oven and burning torch." God always keeps His word, but in this case He goes a step further by making and confirming this covenant with Abram about his descendants and the land of Canaan.