Genesis 17:4

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.

The promise here is introduced with solemnity: "As for me," says the great God, "behold, behold and admire it, behold and be assured of it, my covenant is with thee;" as before (Gen 17:2), I will make my covenant. Note, The covenant of grace is a covenant of God's own making; this he glories in (as for me), and so may we. Now here, I. It is promised to Abraham that he should be a father of many nations; that is, 1. That his seed after the flesh should be very numerous, both in Isaac and Ishmael, as well as in the sons of Keturah: something extraordinary is doubtless included in this promise, and we may suppose that the event answered to it, and that there have been, and are, more of the children of men descended from Abraham than from any one man at an equal distance with him from Noah, the common root. 2. That all believers in every age should be looked upon as his spiritual seed, and that he should be called, not only the friend of God, but the father of the faithful. In this sense the apostle directs us to understand this promise, Rom 4:16, Rom 4:17. He is the father of those in every nation that by faith enter into covenant with God, and (as the Jewish writers express it) are gathered under the wings of the divine Majesty. II. In token of this his name was changed from Abram, a high father, to Abraham, the father of a multitude. This was, 1. To put an honour upon him. It is spoken of as the glory of the church that she shall be called by a new name, which the mouth of the Lord shall name, Isa 62:2. Princes dignify their favourites by conferring new titles upon them; thus was Abraham dignified by him that is indeed the fountain of honour. All believers have a new name, Rev 2:17. Some think it added to the honour of Abraham's new name that a letter of the name Jehovah…

17:4-5 God guaranteed his promise by changing Abram’s name. Abram referred to his noble lineage, as Terah was the “exalted father” (11:27). His new name, Abraham (Hebrew ’ab hamon) was a wordplay on the promise of his own progeny (see also John 8:31-59; Rom 4:16-17; Gal 3:7, 15-19, 29). Whenever the new name was used, he and his household would remember that a multitude of nations would issue from him.