Genesis 18:17

And the Lord said, with His Word, I cannot hide from Abraham that which I am about to do; and it is right that before I do it, I should make it known to him. For

AND THE ETERNAL SAID. I.e., to the host of heaven standing by Him, or to the angel messengers.
It is possible that the word amar (He said) refers to thought, meaning that He thought He should not keep it hidden from Abraham on account of these reasons. Similarly: I ‘said,’ in the noontide of my days I shall go; And he ‘said’ to slay David. Likewise all expressions of speaking within the heart refer to thought., further, 27:41. See Ramban there.

המכסה אני מאברהם, the fact that I am about to overturn Sodom and its satellite towns. G’d reasoned seeing that Avraham was sure to tell his descendants of G’d’s promise to him to give the whole land of Canaan to his descendants, He would diminish these descendants’ inheritance by utterly destroying these towns and the valley which were part of area promised to the Jewish people as an inheritance. He therefore felt duty-bound to obtain Avraham’s consent of what He was about to do. We had learned in Genesis 10,19 that the boundaries of the Canaanite extended to regions that included the Jordan valley. G’d considered it inappropriate to deny Avraham fulfillment of part of His promise by making these parts of the land of Canaan totally useless to its inhabitants.

ואברהם הולך..וה' אמר. He was constantly busy with good deeds and the fulfillment of G’d’s commandments, such as to see his guests off and accompany them beyond the entrance of his house. This is the meaning behind the words למען אשר יצוה את בניו וג'; if Avraham saw that G’d had revealed His plans for Sodom to him, immediately after he accompanied the angels, he would understand that the performance of such kind deeds resulted in one’s gaining an ever more intimate relationship with G’d. He would be encouraged to instruct his children, etc., to act in this manner [pointing out positive results which would accrue to them as a result. Ed.] This is what is meant in Avot 4,2 שכר מצוה מצוה, “the reward of performing a mitzvah is .that one will be encouraged to perform another mitzvah.

המכסה אני, it is appropriate that I do not conceal from Avraham my attribute of goodness; I will inform him that if among all these wicked people in Sodom I find even a quorum of 10 righteous people, this will represent hope that eventually all or most of these people will become penitents. In that event I would bend the scales of justice in favour of the town, giving them an extension of time before destroying them. G’d teaches that He is more interested in the return of sinners to the fold than in administering retribution to them.

וה' אמר, the Lord, Who had appeared to Him initially to inform him of the sins of the Sodomites (13,13) informed Avraham now, after the departure of the angels, about His plans in detail. The Torah adds, in its narrative, what had prompted G’d to take this unusual step of informing Avraham of His plans, seeing that his own safety was not involved. G’d felt duty-bound to apprise Avraham of His intentions.

And it is stated with an expression of wondering: "Shall I conceal from Avraham?" And there are seven reasons for this. The first is because his children would multiply and inherit the plain and the cities, I mean to say Sodom and its satellites... And He stipulated about them that they would inherit it and dwell in it. Hence He said (Genesis 18:18), "Avraham is indeed to become a great nation." And [on account] of the destruction that He threatened about it [and he requested mercy upon it] in his prayer. And that is why He said, "and through him shall be blessed all the nations of the world." And because he was a prophet; and the merit of a prophet is great concerning the [counsels of the Holy One, blessed be He]. And because of that, He said, "For I have known him," the explanation of which is, for the times that I will know him (so is the yod vocalized, with a hiriq). And the fourth is because he fears the Lord, and the merit of those who fear the Lord is great. For 'the counsel of the Lord is to those who fear Him.' And because of this, He said, "in order that he commands." And the fifth is because he is just, and the merit of the just is great in the counsel of the Lord. It is as it is written (Proverbs 3:32), "and His counsel is with the just." And the sixth is in order to enlarge his reward because of his effort to save people and to have them repent... [He set up for them] great estimation, and it is as I am about to elucidate in, "I will now descend" (Genesis 18:21), and as we explained concerning the people of the Flood and the people of the Tower.

המכסה אני...ואברהם היו יהיה לגוי גדול, “how can I conceal from Avraham...seeing that he will develop into a great and important nation?” G–d would not have revealed the future to Avraham except for the fact that he had a future way beyond his physical death. What difference would it then have made if he knew about the fate of Sodom and what had caused it? Seeing that Avraham would become a witness to My exacting retribution from sinners, (even when I do not bring on another deluge) he will be not only be telling his offspring about it, but use his first hand knowledge as a reason to impress them with the fact that G–d not only knows what goes on here on earth, but reacts to it appropriately at a time of His own choosing. Only by paying heed to that knowledge will they live to inherit the land of the Canaanites who will then share the fate of Sodom more or less, [their land not being destroyed. Ed.]

Unless his descendants were to practice justice and righteousness, they would not inherit that land.