Synopsis of the Books of the Bible, by John Nelson Darby, [1857-62], at sacred-texts.com
In chapter 17 we have a fresh revelation of the Lord to Abram, and, I think, are upon higher and holier ground. It is not here calling, or worship, or the world and victory over it in Lot (12-14 [See Note #1] ), or a revelation by the word of how God would accomplish His earthly promises, and what His people should go through (15) not what God was for Abram, but what He was Himself. It is not, I am thy shield and thine exceeding great reward; but I am God Almighty. This is not all He was, but it is what He was His own name; and Abram is called upon to walk correspondently to this name. Hence, also, he does not worship or request anything from God, however high the privilege, but Elohim talks with him. The various parts of His purposes are unfolded, and what Abram is to be before Him in whom he believed. It is the starting-point of God's history of His connection with, and ways in, the world, Jew and Gentile starting from His original sovereign title. That which brings in the Gentiles as well as Israel is before us. It is not the individual seed of promise, as in chapter 22, to which the promise of chapter 12 was confirmed, but the title of God with the first vessels of promise as root of a people set apart to God. In general God's covenant was with him. It is not a legal binding, but a free engagement of God in grace, according to His own mind, that Abraham should be the father of many nations. It is in three parts. God would be a God to Abraham, and to his seed after him; the land wherein he was a stranger is to be to him and to his seed after him; and nations and kings should come out of him.
All these promises are without condition; but principles are set forth binding on Abraham, and expressive of the character of those who enjoy the privileges of God circumcision and free sovereign promise. Circumcision in contrast with law (see Joh 7:22), but expressive of the death of the flesh (compare Rom 4:10-13) [See Note #2], and next, the promise of the seed is given; but this when Abraham, as to the body, was now dead; and as the character of circumcision was peremptory for flesh cannot have to say to God in light so was it as to the promise; it was to the son of promise. Though God might outwardly bless the seed according to flesh, the covenant was exclusively with the heir of promise. Death of flesh (for we are away from God), and simple sovereign grace, are peremptory. The barren woman must be the mother of thousands. Abraham rejoices in the promise, and acts obediently in the order of God.
There is another element here, a common one to this purport in scripture; God's giving a name to Abram and to Sarai also. It signifies the title of direct authority, and entering into relationship on this ground. So Adam, so Pharaoh, so Nebuchadnezzar. Here God having revealed His own name gives one to Abram in connection with Himself. Thenceforth He is the God of Abraham, revealing Abraham's place, and the sign of the covenant in separation to Himself too; Abraham is the father of many nations; Ishmael even is preserved and blessed; but the promised seed stands alone, also has his name (laughter), the child of mere promise of her whom God named too, intimating, though not revealing, resurrection (compare Rom 4:19-22). For this world, Israel as to promise holds the place of Sarah thus named, but when dead according to the flesh. [See Note #1] In chapter 12 it is the path of faith, though with failing, that failing the not owning the separated relationship of God's people (the church) to the heir of the world. Then chapters 13, 14 the believer in a worldly place taken as his portion, the victory of the separated ones, the faith which would not take a shoe-latchet. Chapter 15 the revelation of a numerous seed and Israel's place. Chapter 16 the attempt to have the promise in flesh Hagar. See Galatians.
I read Verse 12 (Gen 17:12) thus: "And father of circumcision [that is, of true separation to God, such as God owns], not only to those of the circumcision, but to those who walk in the steps of the faith of Abraham, which he had being yet uncircumcised." That is, God recognises them (believers from among the Gentiles) as being truly circumcised.