Abraham Pleads for Sodom and Gomorrah - the Destruction (Genesis 18:1 to Genesis 19:38).
‘And Yahweh appeared to him by the oaks of Mamre as he sat in the tent door on the heat of the day'.
Reference to Abraham as ‘him', applied from the last chapter, shows that the covenant accounts have not been inter-connected without thought. It is clear that the site at the oaks of Mamre was the permanent site from which the tribe still operated (compare Genesis 13:18).
“He sat in the tent door at the heat of the day”. He was probably enjoying his siesta under some kind of cover and this was why he spotted the strangers. There is a deliberate contrast between Abraham who sits in the door of his tent, and Lot who sits in the gate of Sodom (Genesis 19:1), bringing out the choices the two men have made.
“Yahweh appeared to him”. It may be that at first he did not realise that the three men he saw coming included Yahweh in human form, perhaps the ‘angel of Yahweh', so the writer lets us know Who it was Who was coming. But the narrative does not tell us when the fact dawned on Abraham. It could however be that it is intended to be indicated by the switch from the impersonal ‘they said' to ‘he said'. That certainly drew attention to the fact that the leader of the three was someone special from Yahweh. Or it could have been when God reveals to him His plans concerning Sodom and Gomorrah. Whatever be the case the reader knows immediately.
The final purpose of their arrival is to bring judgment on Sodom and Gomorrah for their evil ways. This emphasises that Yahweh is ‘the Judge of all the earth' (Genesis 18:25), not just of the tribe. The other two were angels who came as witnesses to demonstrate that the cities were being given a fair chance (Genesis 18:21-22; Genesis 19:1 on).
But the main purpose of the coming of Yahweh Himself is the confirmation of the covenant in respect of a son by Sarah, and, as we learn later, to give Abraham opportunity to intercede on behalf of any righteous people in the guilty cities. It this renewal of the covenant and the promise Abraham received about the cities which makes the writing down of the narrative necessary. The first is the treasured promise of a natural heir. The One Who can destroy Sodom and Gomorrah can surely produce an heir. The second is a record of Yahweh's covenant with Abraham which will result in the deliverance of his nephew, Lot.
The fact that Yahweh comes to inform Abraham of what He is about to do, and that He allows him to be an intercessor (one who goes between) emphasises Abraham's unique position in God's sight. As the beginning of the new people of God he is introduced to God's secrets, and given his first opportunity to influence wider events through intercession.