Or HaChaim on Genesis 18:18/אור החיים על בראשית י״ח:י״ח

1 ואברהם היו יהיה לגוי גדול. "For Abraham is going to be a great nation." Therefore I have to inform him that contrary to what he had thought, righteous people can save both themselves and the towns in which they live. This is the meaning of the last words in our verse, ונברכו בו כל גויי הארץ. The word בו refers back to the words "a great nation."

2One may ask: "of what use is this promise to the nation that will follow Abraham?" G'd answers: "For I am aware that he will instruct his descendants, etc." G'd knew that Abraham would endeavour to teach his children to be good and to do good and to inform them about the ways of the Lord.

3למען הביא ה׳ על אברהם, in order that G'd would be able to fulfil for Abraham, etc. G'd announced here that the essential part of His fondness was not for Abraham's descendants but His fondness of Abraham personally. G'd's planning focused on how to ensure that He could fulfil all the promises He had made to Abraham concerning his descendants. Unless Abraham's descendants observed the paths of G'd this could not be guaranteed.