Taking the accounts of Genesis chapters 15, 17 and 18 together we can see the elements of a covenant being formed between Abraham and God. The following are observations regarding this unfolding covenant.
In chapter fifteen we find that Abram attempts to circumvent God's plan for an heir by choosing his steward. God appears to Abram and ask him if he can number the stars. Such would be the number of his descendants from the promise that God had made to Abram. From Abram and Sarai would come the promise, not by any other means. Abram then asked God: "Lord GOD, how shall I know that I will inherit it?" What we now see is God making a covenant with Abram.
Previous to this, in verse seven, God states that He was the one who had brought him out of his home land to the land where Abram now was dwelling. This is what is known as the preamble of a covenant. God then promises to Abram that from his own body would come an heir that would produce descendants beyond number. As a means of establishing this covenant, God requires that Abram prepare certain animal sacrifices.
The animals were divided, expect for the birds. This was typical of covenants of that day. An animal was used in sacrifice to seal or ratify the covenant. The body of the animal was divided in two parts and then those who were coming into covenant would pass between the divided parts. This was symbolic of their pledge to faithfulness to the covenant. It symbolized that if any of the ones who passed between the parts broke the covenant, then what happened to the sacrifice would also happen to that person who broke covenant.
Darkness fell and Abram had to protect the sacrificed animals. When darkness came, Abram fell into a sleep. God then prophesied to Abram about his descendants and their bondage in Egypt. God also prophesied that He would deliver them after 400 years. Next, we find that God passed between the divided parts of the sacrifice. This is very significant. God was making the covenant to Abram unconditionally. Abram was only the recipient of the covenant, not a part of the bonding of the covenant. God's promises are sure and faithful. God demonstrated before Abram that He would be faithful to that promise. In this we find that Abram believed God's promise and this was put to Abram's charge for righteousness.
In chapter seventeen we have the next step in the process of God's fulfilling the covenant with Abram. In verse one we find a new name for God - El Shaddai, or the God of all supply. In this verse we have God commanding that Abram walk before Him in obedience. Notice this verse:
Genesis 17:1 When Abram was ninety-nine years old, the Lord appeared to Abram and said to him, "I am Almighty God; walk before Me and be blameless."
In this chapter we learn that Abram's name is now changed to Abraham and Sarai's name is changed to Sarah. God again reaffirms the covenant with Abraham and his descendants. In this chapter we have instituted the sign of the covenant - circumcision. All males must be circumcised, otherwise they would not be in covenant with God. Again, this shows froth the nature of covenants of that day and time. There was usually a sign of covenant relationship between those who were in covenant. The difference between this and chapter fifteen is that the covenant is now between God and Abraham and his descendants. By taking on circumcision as the sign of the covenant, those who were circumcised were pledging their faithfulness to that covenant.
In circumcision we can see the nature of the covenant as well. Circumcision was the removing of the foreskin of the flesh. This was a sacrifice made by blood. All sacrifices for covenants were blood sacrifices. When Moses began his return to Egypt, his sons had to be circumcised, otherwise he would have died. His wife, throwing the foreskins of his sons at his feet declared that he was a husband of blood (Ex. 4:25). Circumcision was not only a sign of the covenant, it was what brought them into covenant with God.
Another factor in this chapter is the commandment from God to Abraham to be faithful. God requires those who are in covenant with Him to be faithful to His commandments. Just possessing circumcision would not save a person. They would have to also be faithful before God. Again, this was part of the covenants of that day and time. God would be faithful and become the God of all supply, if those who were in covenant with Him would obey His commandments. If any broke covenant, it would not be God.
Finally, in chapter eighteen we find the climax to the covenant established between God and Abraham. In the first verse we learn that the LORD, or God appeared before Abraham in the planes of Mamre. This is the only time in all of scripture that we find Jehovah God every taking on human form and appearing before a human being. It must be clearly stated that this was not God's true appearance, because no man has ever witnessed God's glory and lived. There was a purpose in this appearance. It possessed two distinct purposes, which were essential for the fulfillment of the covenant between God and Abraham.
First, God and Abraham shared in a covenant meal. In those days, whenever a covenant was established, after the sacrifice was offered, those who had joined in covenant would eat a meal to seal or bond their fellowship in that covenant. This is often called a covenant of salt. In such a meal, the parties who were in covenant would share in that meal and dedicate their pledge to be faithful to that covenant. They would, in effect, renew their faithfulness to the promises of the covenant.
At this time, Sarah had not borne the promised heir. In this chapter we find that the Lord renews that promise and states that in the following year Sarah would give birth to a son and his name would be Isaac. Abraham did not believe this, neither did Sarah. They both laughed at the prospect of having a child. Both Abraham and Sarah understood that it would be impossible for them, on their own, to have a child. If a child were to be born, it would be by God's power and Grace. God was fulfilling His part of the covenant. This required God's presence before Abraham to insure that that pledge would be fulfilled.
With Abraham there was a progress in the unfolding of God's covenant. Each part lead to the development of the whole. With Abraham it took many years to learn to put his full trust and faith in God's promises. He had to learn, by experience, that God was faithful to His word. In time, Abraham was able to so completely put his faith in God's promises that when called upon to sacrifice this son of promise, he was willing to follow God's command without hesitation. He knew that God had promised that in Isaac the promises would be fulfilled. Paul, later, in Hebrews chapter eleven spoke of this faith in God's promises:
Hebrews 11:17-19 By faith Abraham, when he was tested, offered up Isaac, and he who had received the promises offered up his only begotten son, of whom it was said, "In Isaac your seed shall be called," concluding that God was able to raise him up, even from the dead, from which he also received him in a figurative sense. NKJV
Today, we are in covenant with God through Christ. God has fulfilled all the aspects of the covenant through His son Jesus Christ. Christ offered up His body as the sacrifice and shed His blood to ratify the covenant. By the resurrection of Christ that covenant became a reality for all the human race. When we come into covenant with Christ, through our circumcision (our baptism, Col. 2:11-13), then we have bound ourselves to faithfulness to that covenant. Christ, then becomes the giver of all spiritual blessings. We must learn, as Abraham, to put our faith in the promises of God and He will richly bless us.