Ancient people used covenants to formalize agreements between parties, whether for political or economic or even social purposes. Usually such covenants were one of two main types: bilateral and unilateral. Bilateral covenants were ancient agreements negotiated between equals or at least each of the parties had input into the agreement, which defined their responsibilities to produce the desired result. The unilateral covenant was different in that it was not negotiated but dictated by the party of higher rank, such as a king or military general. The covenants God made with Abraham were unilateral covenants. Each time the text reveals that it was God who both initiated the covenant and dictated the conditions whereby Abraham would enjoy the promises God made to him. (Genesis 15:1-18; 17:1-14).
God made two covenants with Abraham. The first was made in Genesis 15 where God promised Abraham both land and descendants. The covenant was unilateral and unconditional. God walked through the animal carcasses alone, showing that he alone was responsible to produce the desired end. Abraham had no input into the covenant, nor did he need to do anything in order to receive its promises, except, perhaps, to believe God, which he did (Genesis 15:6).
In Genesis 17, however, God dictated what Abraham and his descendants must do in order to receive the promise he made there. The covenant was one of intimacy with God, and, if the covenant was obeyed, it would cause the whole world to recognize Abraham and his descendants as the people of God. It was, therefore, a conditional covenant. God was not required to recognize anyone in a special manner if each other participant didn’t fulfill his own part in the covenant. This second covenant by no means negated the first. God was still unconditionally obligated to bless Abraham and his descendents with land, but in response to this God initiated the second covenant of circumcision, a covenant of intimacy and one that identified the participant with God. If Abraham or his descendants wished to be identified with God, they must be circumcised. Only in circumcision is one identified with God, or, put another way, only in this manner will God place his name upon the participant, and thereby claim that one as his own.
Circumcision was the sign of the covenant that identified that one as belonging to God (cp. Exodus 12:48; Deuteronomy 30:6 and Jeremiah 4:4). God used other signs for other covenants. For example, God gave Noah the rainbow as a sign he would never again flood the whole earth; and in Exodus 13:7-10 unleavened bread is given to Israel as a sign between them and God recalling the day he made them a nation, calling them out of the land of Egypt where they had been slaves. As for the covenant of circumcision, the cutting off to one’s flesh implied one cutting oneself off from the rest of mankind. One identified with God, not the world. It was a physical sign but meant to point to one’s heart (Deuteronomy 10:14-17). In the end it was a covenant of the heart, a covenant of intimacy with God, a covenant that told the world God is my God, and I am his son.