God cut a covenant with Abraham.
Genesis 15 tells the story of GOD again speaking to Abraham and, this time, promising Abraham he would have a son and that Abraham's seed would become as numerous as the stars. Abraham asked for a sign of this promise. Look at what GOD did:
1 After these things the Word of the Lord came to Abram in a vision, saying, “Do not be afraid, Abram. I am your shield and will be your exceedingly great reward.” 2 But Abram said, “Lord, what will You give me, seeing I go childless, and the heir of my house is Eliezer of Damascus, the son of Masek, my domestic maidservant?” 3 Then Abram said, “Look, You have given me no offspring; indeed, my household servant is my heir.”
4 And immediately the Voice of the Lord came to him, saying, “This one shall not be your heir, but the One who will come from your own body shall be your heir.” 5 Then He brought him outside and said to him, “Look now toward heaven, and count the stars if you are able to number them.” And He said to him, “So shall your seed be.” 6 And Abram believed God, and He accounted it to him for righteousness. 7 Then He said to him, “I am the God who brought you out of Ur of the Chaldeans, to give you this land to inherit it.” 8 And he said, “Master and Lord, how will I know I will inherit it?” 9 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.”
10 So he brought all these to Him and cut them in half, down the middle, and placed each piece opposite the other; but he did not cut the birds in two. 11 And when the vultures came down on the carcasses, Abram drove them away.
12 Now around sunset, a trance fell upon Abram; and behold, horror and great darkness fell upon him. 13 Then He said to Abram, “Know for certain that your seed will be strangers in a land not their own, and will serve them; and they will afflict and humble them four hundred years. 14 Also, the nation they serve I will judge; and afterward, they shall come out with great possessions. 15 But as for you, you shall go to your fathers in peace, buried in a good old age. 16 Then in the fourth generation they shall return here, for the sin of the Amorites is not yet filled up.”
17 And it came to pass, when the sun went down, that there was a flame, and behold, there appeared a smoking oven and lamps of fire that passed between those divided pieces. 18 On the same day, the Lord made a covenant with Abram, saying, “I will give this land to your seed, from the river of Egypt to the great river, the River Euphrates— 19 the Kenites, the Kenezzites, the Kadmonites, 20 the Hittites, the Perizzites, the Rephaim, 21 the Amorites, the Canaanites, the Euaites, the Girgashites, and the Jebusites.”
(Genesis 15:1-21 OSB)
For the modern reader, this miracle may seem interesting but not overly significant.
But it is significant!
When ancient people made a covenant (a verbal contract, basically), they would seal the agreement through a meal. The meal would be divided in half, placed on either side of a walkway. Once the terms of the covenant were agreed upon, the parties would walk between the divided meal preparations as a sign that the deal was settled.
In fact, the ancient Hebrew word for covenant (כָּרַת) literally refers to cutting. Thus, the saying "cut a deal."
So, what GOD really did here, was to use the ancient human custom with which Abraham was familiar in order to show that the deal was sealed. GOD miraculously passed between the divided parts of the heifer and goat using the lamps of fire to reveal the action to Abraham.
This meal-to-seal-the-deal covenant practice is seen again in subsequent covenants GOD made as recorded in later Scripture.
(You can read more about these things online, such as here and here.)
Biblical Principle #74: Abraham's Seed are the Faithful