A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
gen 18:1ENTERTAINMENT OF ANGELS. (Gen 18:1-8)
the Lord appeared--another manifestation of the divine presence, more familiar than any yet narrated; and more like that in the fulness of time, when the Word was made flesh.
plains of Mamre--rather, terebinth or oak of Mamre; a tall-spreading tree or grove of trees.
sat in the tent door--The tent itself being too close and sultry at noon, the shaded open front is usually resorted to for the air that may be stirring.
gen 18:2lift up his eyes . . . and, lo, three men--Travellers in that quarter start at sunrise and continue till midday when they look out for some resting-place.
he ran to meet them--When the visitor is an ordinary person, the host merely rises; but if of superior rank, the custom is to advance a little towards the stranger, and after a very low bow, turn and lead him to the tent, putting an arm round his waist, or tapping him on the shoulder as they go, to assure him of welcome.
gen 18:3My Lord, if now I have found favor--The hospitalities offered are just of the kind that are necessary and most grateful, the refreshment of water, for feet exposed to dust and heat by the sandals, being still the first observed among the pastoral people of Hebron.
gen 18:5for therefore are ye come--No questions were asked. But Abraham knew their object by the course they took--approaching directly in front of the chief sheik's tent, which is always distinguishable from the rest and thus showing their wish to be his guests.
gen 18:6Abraham hastened . . . unto Sarah . . . make cakes upon the hearth--Bread is baked daily, no more than is required for family use, and always by the women, commonly the wife. It is a short process. Flour mixed with water is made into dough, and being rolled out into cakes, it is placed on the earthen floor, previously heated by a fire. The fire being removed, the cakes are laid on the ground, and being covered over with hot embers, are soon baked, and eaten the moment they are taken off.
gen 18:7Abraham ran unto the herd, and fetched a calf--Animal food is never provided, except for visitors of a superior rank when a kid or lamb is killed. A calf is still a higher stretch of hospitality, and it would probably be cooked as is usually done when haste is required--either by roasting it whole or by cutting it up into small pieces and broiling them on skewers over the fire. It is always eaten along with boiled corn swimming in butter or melted fat, into which every morsel of meat, laid upon a piece of bread, is dipped, before being conveyed by the fingers to the mouth.
gen 18:8milk--A bowl of camel's milk ends the repast.
he stood by them under the tree--The host himself, even though he has a number of servants, deems it a necessary act of politeness to stand while his guests are at their food, and Abraham evidently did this before he was aware of the real character of his visitors.
gen 18:10REPROOF OF SARAH. An inquiry about his wife, so surprising in strangers, the subject of conversation, and the fulfilment of the fondly cherished promise within a specified time, showed Abraham that he had been entertaining more than ordinary travellers (Heb 13:2). (Gen 18:9-15)
Sarah heard it in the tent door, which was behind him--The women's apartment is in the back of the tent, divided by a thin partition from the men's.
gen 18:12Therefore Sarah laughed within herself--Long delay seems to have weakened faith. Sarah treated the announcement as incredible, and when taxed with the silent sneer, she added falsehood to distrust. It was an aggravated offense (Act 5:4), and nothing but grace saved her (Rom 9:18).
gen 18:16DISCLOSURE OF SODOM'S DOOM. (Gen 18:16-22)
the men rose . . . Abraham went with them--It is customary for a host to escort his guests a little way.
gen 18:17the Lord said, Shall I hide--The chief stranger, no other than the Lord, disclosed to Abraham the awful doom about to be inflicted on Sodom and the cities of the plain for their enormous wickedness.
gen 18:21I will go down . . . and see--language used after the manner of men. These cities were to be made examples to all future ages of God's severity; and therefore ample proof given that the judgment was neither rash nor excessive (Eze 18:23; Jer 18:7).
gen 18:23ABRAHAM'S INTERCESSION. (Gen 18:23-33)
Abraham drew near, and said, &c.--The scene described is full of interest and instruction--showing in an unmistakable manner the efficacy of prayer and intercession. (See also Pro 15:8; Jam 5:16). Abraham reasoned justly as to the rectitude of the divine procedure (Rom 3:5-6), and many guilty cities and nations have been spared on account of God's people (Mat 5:13; Mat 24:22).
gen 18:33the Lord . . . left communing . . . and Abraham returned unto his place--Why did Abraham cease to carry his intercessions farther? Either because he fondly thought that he was now sure of the cities being preserved (Luk 13:9), or because the Lord restrained his mind from further intercession (Jer 7:16; Jer 11:14). But there were not ten "righteous persons." There was only one, and he might without injustice have perished in the general overthrow (Ecc 9:2). But a difference is sometimes made, and on this occasion the grace of God was manifested in a signal manner for the sake of Abraham. What a blessing to be connected with a saint of God!