Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
The covenant had been made with Abram for at least fourteen years, and yet Abram remained without any visible sign of its accomplishment, and was merely pointed in faith to the inviolable character of the promise of God. Jehovah now appeared to Him again, when he was ninety-nine years old, twenty-four years after his migration, and thirteen after the birth of Ishmael, to give effect to the covenant and prepare for its execution. Having come down to Abram in a visible form (Gen 17:22), He said to him, "I am El Shaddai (almighty God): walk before Me and be blameless." At the establishment of the covenant, God had manifested Himself to him as Jehovah (Gen 15:7); here Jehovah describes Himself as El Shaddai, God the Mighty One. שׁדּי: from שׁדד to be strong, with the substantive termination ai, like חגּי the festal, ישׁישׁי the old man, סיני the thorn-grown, etc. This name is not to be regarded as identical with Elohim, that is to say, with God as Creator and Preserver of the world, although in simple narrative Elohim is used for El Shaddai, which is only employed in the more elevated and solemn style of writing. It belonged to the sphere of salvation, forming one element in the manifestation of Jehovah, and describing Jehovah, the covenant God, as possessing the power to realize His promises, even when the order of nature presented no prospect of their fulfilment, and the powers of nature were insufficient to secure it. The name which Jehovah thus gave to Himself was to be a pledge, that in spite of "his own body now dead," and "the deadness of Sarah's womb" (Rom 4:19), God could and would give him the promised innumerable posterity. On the other hand, God required this of Abram, "Walk before Me (cf. Gen 5:22) and be blameless" (Gen 6:9). "Just as righteousness received in faith was necessary for the establishment of the covenant, so a blameless walk before God was required for the maintenance and confirmation of the covenant." This introduction is followed by a more definite account of the new revelation; first of the promise involved in the new name of God (Gen 17:2-8), and then of the obligation imposed upon Abram (Gen 17:9-14). "I will give My covenant," says the Almighty, "between Me and thee, and multiply thee exceedingly." בּרית נתן signifies, not to make a covenant, but to give, to put, i.e., to realize, to set in operation the things promised in the covenant - equivalent to setting up the covenant (cf. Gen 17:7 and Gen 9:12 with Gen 9:9). This promise Abram appropriated to himself by falling upon his face in worship, upon which God still further expounded the nature of the covenant about to be executed.
On the part of God אני placed at the beginning absolutely: so far as I am concerned, for my part) it was to consist of this: (1) that God would make Abram the father (אב instead of אני chosen with reference to the name Abram) of a multitude of nations, the ancestor of nations and kings; (2) that He would be God, show Himself to be God, in an eternal covenant relation, to him and to his posterity, according to their families, according to all their successive generations; and (3) that He would give them the land in which he had wandered as a foreigner, viz., all Canaan, for an everlasting possession. As a pledge of this promise God changed his name אברם, i.e., high father, into אברהם, i.e., father of the multitude, from אב and רהם, Arab. ruhâm = multitude. In this name God gave him a tangible pledge of the fulfilment of His covenant, inasmuch as a name which God gives cannot be a mere empty sound, but must be the expression of something real, or eventually acquire reality.
On the part of Abraham (ואתּה thou, the antithesis to אני, as for me, Gen 17:4) God required that he and his descendants in all generations should keep the covenant, and that as a sign he should circumcise himself and every male in his house. המּול Niph. of מוּל, and נמלתּם perf. Niph. for נמלּתם, from מלל = מוּל. As the sign of the covenant, circumcision is called in Gen 17:13, "the covenant in the flesh," so far as the nature of the covenant was manifested in the flesh. It was to be extended not only to the seed, the lineal descendants of Abraham, but to all the males in his house, even to every foreign slave not belonging to the seed of Abram, whether born in the house or acquired (i.e., bought) with money, and to the "son of eight days," i.e., the male child eight days old; with the threat that the uncircumcised should be exterminated from his people, because by neglecting circumcision he had broken the covenant with God. The form of speech ההיא הנּפשׁ נכרתה, by which many of the laws are enforced (cf. Exo 12:15, Exo 12:19; Lev 7:20-21, Lev 7:25, etc.), denotes not rejection from the nation, or banishment, but death, whether by a direct judgment from God, an untimely death at the hand of God, or by the punishment of death inflicted by the congregation or the magistrates, and that whether יוּמת מות is added, as in Exo 31:14, etc., or not. This is very evident from Lev 17:9-10, where the extermination to be effected by the authorities is distinguished from that to be executed by God Himself (see my biblische Archologie ii. 153, 1). In this sense we sometimes find, in the place of the earlier expression "from his people," i.e., his nation, such expressions as "from among his people" (Lev 17:4, Lev 17:10; Num 15:30), "from Israel" (Exo 12:15; Num 19:13), "from the congregation of Israel" (Exo 12:19); and instead of "that soul," in Lev 17:4, Lev 17:9 (cf. Exo 30:33, Exo 30:38), we find "that man."
The appointment of the sign of the covenant was followed by this further revelation as to the promised seed, that Abram would receive it through his wife Sarai. In confirmation of this her exalted destiny, she was no longer to be called Sarai (שׂרי, probably from שׂרר with the termination ai, the princely), but שׂרה, the princess; for she was to become nations, the mother of kings of nations. Abraham then fell upon his face and laughed, saying in himself (i.e., thinking), "Shall a child be born to him that is a hundred years old, or shall Sarah, that is ninety years old, bear?" "The promise was so immensely great, that he sank in adoration to the ground, and so immensely paradoxical, that he could not help laughing" (Del.). "Not that he either ridiculed the promise of God, or treated it as a fable, or rejected it altogether; but, as often happens when things occur which are least expected, partly lifted up with joy, partly carried out of himself with wonder, he burst out into laughter" (Calvin). In this joyous amazement he said to God (Gen 17:18), "O that Ishmael might live before Thee!" To regard these words, with Calvin and others, as intimating that he should be satisfied with the prosperity of Ishmael, as though he durst not hope for anything higher, is hardly sufficient. The prayer implies anxiety, lest Ishmael should have no part in the blessings of the covenant. God answers, "Yes (אבל imo), Sarah thy wife bears thee a son, and thou wilt call his name Isaac (according to the Greek form Ἰσαάκ, for the Hebrew יצחק, i.e., laughter, with reference to Abraham's laughing; Gen 17:17, cf. Gen 21:6), and I will establish My covenant with him," i.e., make him the recipient of the covenant grace. And the prayer for Ishmael God would also grant: He would make him very fruitful, so that he should beget twelve princes and become a great nation. But the covenant, God repeated (Gen 17:21), should be established with Isaac, whom Sarah was to bear to him at that very time in the following year. - Since Ishmael therefore was excluded from participating in the covenant grace, which was ensured to Isaac alone; and yet Abraham was to become a multitude of nations, and that through Sarah, who was to become "nations" through the son she was to bear (Gen 17:16); the "multitude of nations" could not include either the Ishmaelites or the tribes descended from the sons of Keturah (Gen 25:2.), but the descendants of Isaac alone; and as one of Isaac's two sons received no part of the covenant promise, the descendants of Jacob alone. But the whole of the twelve sons of Jacob founded only the one nation of Israel, with which Jehovah established the covenant made with Abraham (Ex 6 and 20-24), so that Abraham became through Israel the lineal father of one nation only. From this it necessarily follows, that the posterity of Abraham, which was to expand into a multitude of nations, extends beyond this one lineal posterity, and embraces the spiritual posterity also, i.e., all nations who are grafted ἐκ πίστεως Ἀβραάμ into the seed of Abraham (Rom 4:11-12, and Rom 4:16, Rom 4:17). Moreover, the fact that the seed of Abraham was not to be restricted to his lineal descendants, is evident from the fact, that circumcision as the covenant sign was not confined to them, but extended to all the inmates of his house, so that these strangers were received into the fellowship of the covenant, and reckoned as part of the promised seed. Now, if the whole land of Canaan was promised to this posterity, which was to increase into a multitude of nations (Gen 17:8), it is perfectly evident, from what has just been said, that the sum and substance of the promise was not exhausted by the gift of the land, whose boundaries are described in Gen 15:18-21, as a possession to the nation of Israel, but that the extension of the idea of the lineal posterity, "Israel after the flesh," to the spiritual posterity, "Israel after the spirit," requires the expansion of the idea and extent of the earthly Canaan to the full extent of the spiritual Canaan, whose boundaries reach as widely as the multitude of nations having Abraham as father; and, therefore, that in reality Abraham received the promise "that he should be the heir of the world" (Rom 4:13).
(Note: What stands out clearly in this promise-viz., the fact that the expressions "seed of Abraham" (people of Israel) and "land of Canaan" are not exhausted in the physical Israel and earthly Canaan, but are to be understood spiritually, Israel and Canaan acquiring the typical significance of the people of God and land of the Lord - is still further expanded by the prophets, and most distinctly expressed in the New Testament by Christ and the apostles. This scriptural and spiritual interpretation of the Old Testament is entirely overlooked by those who, like Auberlen, restrict all the promises of God and the prophetic proclamations of salvation to the physical Israel, and reduce the application of them to the "Israel after the spirit," i.e., to believing Christendom, to a mere accommodation.)
And what is true of the seed of Abraham and the land of Canaan must also hold good of the covenant and the covenant sign. Eternal duration was promised only to the covenant established by God with the seed of Abraham, which was to grow into a multitude of nations, but not to the covenant institution which God established in connection with the lineal posterity of Abraham, the twelve tribes of Israel. Everything in this institution which was of a local and limited character, and only befitted the physical Israel and the earthly Canaan, existed only so long as was necessary for the seed of Abraham to expand into a multitude of nations. So again it was only in its essence that circumcision could be a sign of the eternal covenant. Circumcision, whether it passed from Abraham to other nations, or sprang up among other nations independently of Abraham and his descendants (see my Archologie, 63, 1), was based upon the religious view, that the sin and moral impurity which the fall of Adam had introduced into the nature of man had concentrated itself in the sexual organs, because it is in sexual life that it generally manifests itself with peculiar force; and, consequently, that for the sanctification of life, a purification or sanctification of the organ of generation, by which life is propagated, is especially required. In this way circumcision in the flesh became a symbol of the circumcision, i.e., the purification, of the heart (Deu 10:16; Deu 30:6, cf. Lev 26:41; Jer 4:4; Jer 9:25; Eze 44:7), and a covenant sign to those who received it, inasmuch as they were received into the fellowship of the holy nation (Exo 19:6), and required to sanctify their lives, in other words, to fulfil all that the covenant demanded. It was to be performed on every boy on the eighth day after its birth, not because the child, like its mother, remains so long in a state of impurity, but because, as the analogous rule with regard to the fitness of young animals for sacrifice would lead us to conclude, this was regarded as the first day of independent existence (Lev 22:27; Exo 22:29; see my Archologie, 63).
When God had finished His address and ascended again, Abraham immediately fulfilled the covenant duty enjoined upon him, by circumcision himself on that very day, along with all the male members of his house. Because Ishmael was 13 years old when he was circumcised, the Arabs even now defer circumcision to a much later period than the Jews, generally till between the ages of 5 and 13, and frequently even till the 13th year.