Covenants may be given or without without obligations. In commenting on Genesis 15 Jon D. Levenson says:
9-11: The ritual of cutting animals in half and passing between them is found both in the Bible and in Mesopotamia. The parallel in Jeremiah 34.17-22 makes it likely that the essence of the ritual is a self-curse: those walking between the pieces will be like the dead animals if they violate the covenant. In the case at hand, remarkably, it is the LORD, symbolized by the "smoking oven" and "flaming torch" (15.17) who invokes the self-curse, and nothing is said about any covenantal obligations Abram is to fulfill. This type of covenant is called a covenant of grant, which is a reward for past loyalty, and does not involve any obligations upon the grantee. The same pattern is prominent in texts about the covenant with David (2 Samuel 7.8-16; Psalm 80.20-37).
Genesis 15 describes a covenant of grant. There are no obligations placed on Abram; it is the same type of covenant God establishes with David. The covenant in Genesis 17 comes with obligations: circumcision. It is possible to side-step the obligation placed on Abraham since circumcision is called a sign. However, the terms of the covenant in Genesis 17 are different for both parties.
In his speech before the Sanhedrin, Stephen states the two are different. He describes the one in Genesis 15 as a promising and Genesis 17 as the covenant of circumcision:
2 And Stephen said: “Brothers and fathers, hear me. The God of glory appeared to our father Abraham when he was in Mesopotamia, before he lived in Haran, 3 and said to him, ‘Go out from your land and from your kindred and go into the land that I will show you.’ 4 Then he went out from the land of the Chaldeans and lived in Haran. And after his father died, God removed him from there into this land in which you are now living. 5 Yet he gave him no inheritance in it, not even a foot's length, but promised to give it to him as a possession and to his offspring after him, though he had no child. 6 And God spoke to this effect—that his offspring would be sojourners in a land belonging to others, who would enslave them and afflict them four hundred years. 7 ‘But I will judge the nation that they serve,’ said God, ‘and after that they shall come out and worship me in this place.’ 8 And he gave him the covenant of circumcision. And so Abraham became the father of Isaac, and circumcised him on the eighth day, and Isaac became the father of Jacob, and Jacob of the twelve patriarchs. (Acts 7 ESV)
Stephen calls the possession of land a promise, ἐπαγγέλλομαι, which the BDAG explains:
ἐπαγγέλλομαι ❶ to declare to do something, with the implication of obligation to carry out what is stated, promise, offer a. of human promises and offers... b. of God: promise
Unlike Genesis 15, which came without obligation, Stephen recognizes circumcision was imposed in Genesis 17. To distinguish between the two, he calls the first ἐπαγγέλλομαι, promise to and the second διαθήκη, covenant.
Stephen's description of the events of Genesis 15 also deviates from the LXX:
In that day the Lord made a covenant with Abram, saying, To thy seed I will give this land, from the river of Egypt to the great river Euphrates. (Genesis 15:18)
ἐν τῇ ἡμέρᾳ ἐκείνῃ διέθετο Κύριος τῷ ῞Αβραμ διαθήκην λέγων· τῷ σπέρματί σου δώσω τὴν γῆν ταύτην, ἀπὸ τοῦ ποταμοῦ Αἰγύπτου ἕως τοῦ ποταμοῦ τοῦ μεγάλου, ποταμοῦ Εὐφράτου
The LXX uses the verb διατίθημι from which διαθήκη, covenant is derived, Stephen replaces διατίθημι with the verb ἐπαγγέλλομαι, the act of making a promise. ἐπαγγέλλομαι is used just twice in the LXX (cf. Esther 4:7, Proverbs 13:12), neither of which describe making a covenant. What Stephen has done is make clear the distinction between the two covenants. The one without obligations he attributes to God's act of promising; the one with obligations he calls the covenant of circumcision.
The difference between the two is significant. Abram's response to what Stephen calls the act of promising is central to the Gospel:
Abraham believed God and it was counted to him as righteousness (Genesis 15:6)
(Romans 4:3, Galatians 3:6, James 2:23)
In addition, it may seem as if Genesis 17 merely repeats God's side of the covenant, land as a possession. However, that requires overlooking the birth of Ishmael which occurred between the two covenants. That is, Ishmael was born under the covenant given in Genesis 15, yet he is excluded from the covenant in Genesis 17. Abraham protests this exclusion:
And Abraham said to God, “Oh that Ishmael might live before you!” (Genesis 17:18)
Abraham understands the covenant just spoken excludes Ishmael and he intercedes on Ishmael's behalf. God responds with "no" but does agree to bless Ishmael:
19 God said, “No, but Sarah your wife shall bear you a son, and you shall call his name Isaac. I will establish my covenant with him as an everlasting covenant for his offspring after him. 20 As for Ishmael, I have heard you; behold, I have blessed him and will make him fruitful and multiply him greatly. He shall father twelve princes, and I will make him into a great nation. 21 But I will establish my covenant with Isaac, whom Sarah shall bear to you at this time next year.” (Genesis 17)
Paul also recognizes there are two different covenants. He also uses Stephen's ἐπαγγέλλομαι (see Galatians and Romans) and he gives this explanation:
21 Tell me, you who desire to be under the law, do you not listen to the law? 22 For it is written that Abraham had two sons, one by a slave woman and one by a free woman. 23 But the son of the slave was born according to the flesh, while the son of the free woman was born through promise. 24 Now this may be interpreted allegorically: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar. 25 Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children. 26 But the Jerusalem above is free, and she is our mother. (Galatians 4)
Paul's use of Hagar and Ishmael makes sense only if we accept Stephen's description of two covenants, one in Genesis 15 the other in Genesis 17; these are known by two different women and two different sons. Ishmael was born under the first covenant; Isaac under the second. Hagar, the slave woman is Sarai's slave, not Abram's. Under the Genesis 15 covenant Ishmael legally belongs to Sarai and is called Abraham's son born according to the flesh. Unless God changed the terms of the covenant, Ishmael is excluded only if Genesis 17 is a different covenant. If there is only one, then God broke the first to exclude Ishmael.
Paul's questionable allegory makes a further allusion to the distinction between the two covenants by calling the first from Mount Sinai in Arabia. Arabia is the land given by God to Ishmael in response to Abraham's request when the second covenant is given.
The covenant of circumcision is the human requirement to the second covenant. Isaac would be born under the second covenant and Ishmael was blessed with land, albeit different from Isaac, because Abraham circumcised him after hearing the second covenant.
1. Jon D. Levenson, The Jewish Study Bible, Edited by Adele Berlin and Marc Zvi Brettler, Oxford University Press, 2004, pp. 35-36
2. Fredrick William Danker, A Greek-English Lexicon of the New Testament and Other Early Christian Literature, The University Chicago Press, 2000, p. 356
3. Another way of distinguishing between the two would be to call them by Abraham's name at the time they were given. Genesis 17 would be the Abrahamic Covenant. Genesis 15 would be the Abramic Covenant.