Abram really wants a son to be the heir to his kingdom, so after Lots rescue God came to Abram in a vision and told him that He will give you a son from your own flesh to inherit all of your possessions. God also said that He will give his descendants all the the land from the River of Egypt (Nile) to the Euphrates and your descendants will be more numerous than the stars. Abram bought a heifer, goat, and a ram; each three years old, a dove, and a young pigeon. He sacrificed these animals so that he could get the land.
Atheist-
First I want to talk about History for a moment: When the Bible was being written (after the Babylonian Exile in 300 BCE) they didn't know a whole lot about astronomy. If God was to be true to his promise of having Abram's descendants more numerous than the stars Abram would need to have billions upon trillions of descendants. To an ancient jewish priest writing the Bible it might only seem to be a few thousand. Although this doesn't seem very important it still shows that God doesn't keep all of his promises, I will speak of more in time.
Next I want to speak of the sacrificing; of course killing is wrong, it says so in the Bible and morals of most people think it's common sense. God told Abram to sacrifice these animals for land, this makes me think that Abram was greedy (Killing for a little land) and it shows that God likes the greedy. Why would sacrificing animals help God give land to Abram? I can only say one thing "Blood for the Blood God".