A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown [1882] at sacred-texts.com
Abram . . . ninety years old and nine--thirteen years after the birth of Ishmael [Gen 16:16]. During that interval he had enjoyed the comforts of communion with God but had been favored with no special revelation as formerly, probably on account of his hasty and blameable marriage with Hagar.
the Lord appeared--some visible manifestation of the divine presence, probably the Shekinah or radiant glory of overpowering effulgence.
I am the Almighty God--the name by which He made Himself known to the patriarchs (Exo 6:3), designed to convey the sense of "all-sufficient" (Psa 16:5-6; Psa 73:25).
walk . . . and . . . perfect--upright, or sincere (Psa 51:6) in heart, speech, and behavior.
Abram fell on his face--the attitude of profoundest reverence assumed by Eastern people. It consists in the prostrate body resting on the hands and knees, with the face bent till the forehead touches the ground. It is an expression of conscious humility and profound reverence.
my covenant is with thee--Renewed mention is made of it as the foundation of the communication that follows. It is the covenant of grace made with all who believe in the Saviour.
but thy name shall be Abraham--In Eastern countries a change of name is an advertisement of some new circumstance in the history, rank, or religion of the individual who bears it. The change is made variously, by the old name being entirely dropped for the new, or by conjoining the new with the old; or sometimes only a few letters are inserted, so that the altered form may express the difference in the owner's state or prospects. It is surprising how soon a new name is known and its import spread through the country. In dealing with Abraham and Sarai, God was pleased to adapt His procedure to the ideas and customs of the country and age. Instead of Abram, "a high father," he was to be called Abraham, "father of a multitude of nations" (see Rev 2:17).
I will give unto thee . . . the land--It had been previously promised to Abraham and his posterity (Gen 15:18). Here it is promised as an "everlasting possession," and was, therefore, a type of heaven, "the better country" (Heb 11:16).
Every man child among you shall be circumcised--This was the sign in the Old Testament Church as baptism is in the New, and hence the covenant is called "covenant of circumcision" (Act 7:8; Rom 4:11). The terms of the covenant were these: on the one hand Abraham and his seed were to observe the right of circumcision; and on the other, God promised, in the event of such observance, to give them Canaan for a perpetual possession, to be a God to him and his posterity, and that in him and his seed all nations should be blessed.
As for Sarai . . . I will . . . give thee a son also of her--God's purposes are gradually made known. A son had been long ago promised to Abraham. Now, at length, for the first time he is informed that it was to be a child of Sarai.
Abraham fell upon his face, and laughed--It was not the sneer of unbelief, but a smile of delight at the improbability of the event (Rom 4:20).
O that Ishmael might live before thee--natural solicitude of a parent. But God's thoughts are not as man's thoughts [Isa 55:8].
The blessings of the covenant are reserved for Isaac, but common blessings were abundantly promised to Ishmael; and though the visible Church did not descend from his family, yet personally he might, and it is to be hoped did, enjoy its benefits.