This chapter is evidently law, but the delay of promise to want of faith gives to flesh the occasion of putting itself on this ground. God uses it to raise the question of righteousness- to judge flesh; it must return back to promise in submission of will. Israel under law, Abraham's seed according to flesh, God does not give up; not only promise was before law, but, for flesh, law must come after promise, because flesh takes up law, in self confidence, to obtain the hope of promise.
Here chapter 15 is the promise; the manner of its accomplishment, in divine grace and power, is not yet revealed, nor is it until God reveals Himself in chapter 17, after law. Christ may be known after the flesh; but as Sarai was the state, no fruit of promise by divine power; yet Abram acts on flesh's impatience to have it, according to flesh's desire, in its own way.
In a certain sense Ishmael answered to chapter 15: 4, but it was all flesh's doing. Under promise, Israel according to flesh will have inheritance, but it is not in the place of Sarah and Abraham, the heavenly glory over the Gentiles; in itself, in chapter 21, it is cast out, and cannot be heir, it will come in no doubt under grace. Even in chapter 15, it was promise according to desire; in chapter 17 according to Elohim's own full purpose, and direct revelation of Himself; chapter 15, we have seen, however, met faith, only Abram did not, in reply, rise above want.
NOTE.- Up to the end of chapter 15, we have promises fully, and a covenant for earthly promises as to Israel, and government, the smoking lamp, and the furnace. But the development of the seed is after the entering into relationship by express revelation, and the consecration of Abraham to Himself by God by circumcision—the judgment, though here only partial, of the flesh; before this we have the earthly seed, which is according to law—Hagar, which God takes care of providentially, but which is not the true personal son of promise in grace.
Note further, this is not the leading to faith by grace; it was the seal of faith, as we have the Holy Ghost as a seal—the special relationship in which Abraham had to walk with God.
It is not until chapter 17: 19 that we have the personal seed; the promises to the seed, and to the land are confirmed, and will surely be accomplished, but the personal seed is nominally revealed to the exclusion of the legal seed, though the former must be born for it to be carried out.
Historically we are on earth, and it goes on so, but in principle we are getting out of flesh—promise and circumcision, or rather circumcision and promise, with known relationship, and communion, by the revelation of God Himself, are brought in. What God is, is made the ground of relationship—hence, note, communion.
Remark further, Lot was never circumcised; circumcision is not simply believing, though it be the true place of every believer now. Israel was circumcised when they had crossed the Jordan, as remarked elsewhere, not in the wilderness. Lot, though he left Ur, was, as to his own faith, not separate from the world, on the contrary, connected himself with it—was a believer in the world. Circumcision, in its full import, takes out of the flesh; we have died with Christ, and cannot be consequently alive in the world.
The reproach of Egypt was rolled away at Gilgal; hence, circumcision comes after chapter 15, which connected Abram with this world, in promise; then we have the heir, as an immediate promise, and the judgment of the world, but God in communion with Abraham about it. This gives the character of chapters a7 and 18; millennial promise may come in, but founded on death and resurrection.