One day, the Lord appeared to Abram in a vision, promising, "Do not fear, Abram, I am a shield to you; your reward will be very great" (Genesis 15:1). Abram wasn't totally sure, since he did not have any children yet, and wondered and was concerned how this promise and the previous promises would come about.
To reassure Abram that He would keep His word to him, God made a covenant. God asks him to find and kill a heifer, a ram, a goat, a dove, and a pigeon. Then, Abram was to cut them in half (except the birds) and lay the pieces in two rows, leaving a path through the center. God put Abram into a deep sleep and a deep darkness descended. God passed through the cut pieces, effectuating the everlasting covenant between God and Abram. A blood covenant communicated a curse with an oath. The parties involved would walk the path between the slaughtered animals so to say, "May this be done to me if I do not keep my oath."
There was an important difference in the blood oath that God made with Abram in Genesis 15. When the evening came, God appeared in the form of a "smoking fire pot and flaming torch [that] passed between the pieces" (Genesis 15:17). But Abram had fallen "into a deep sleep, and a thick and dreadful darkness came over him". Thus, GOD ALONE passed through the pieces of dead animals, and the covenant was sealed by God alone.
Abram expected to walk down the middle path but God could not let Him have any part in this Covenant. There was not part for Abram to have, because the covenant did not depend on Abram at all. The truth is that God was making a covenant that concerned Abram - not only to be fully responsible for His part but also to be fully responsible to fulfill Abram's part for him. Everything depended on God, who promised to be faithful to His covenant. "When God made this covenat, since there was no one greater for Him to swear by, He swore by Himself". Now Abram and his descendants (us) could trust, count on, and believe in everything God promised.
Normally, the two parties would walk between the pieces as they committed to the covenant, with the implicit warning being that a failure to keep the promise would bring the same fate upon the promise-breaker as was brought upon the animals - death. Here, however, only God passes between the animal pieces. The Lord takes it upon Himself alone to accomplish His word, and vows to be destroyed should He not keep it.
In this Covenant God is basically telling Abram, "I'm putting My very deity on the line here. I'm swearing to you by MY holy nature. If I don't keep this word, I will no longer be God." Then He confirms the covenant by walking down the middle path between the dead animals representing Abram and HIMSELF. So not only did He swear that He would fulfill His own part concerning Abram but He also swore that He would fulfill Abraham's part also. This made God totally responsible to bring both parts to pass. Now this covenant was not just to Abram but to all of God's people. He made a covenant with the promise that He seals with an oath based upon His own very nature. There is no conceivably higher guarantee than that.
Then God renamed abram. "No longer shall your name be called Abram, but your name shall be Abraham; for I have made you a father of many nations." Gen 17:5
