וירא אליו ה' באלוני ממרא - Hashem appeared to (Avraham) him in Elonei Mamrei. This revelation of Hashem is unique b/c we are not told about the content and nature of this revelation, just that Hashem appeared to Avraham. Only after the story with the guests, Hashem speaks to Avraham [18-13]. First "seeing" and then speech. When it comes to Moshe we learn that Hashem spoke to him and he asks for "seeing" - "הראיני נא את כבודך" - "Show me Your Honor". And his request is not accepted - "לא תוכל לראות את פני כי לא יראני האדם וחי". 

To understand the difference between Moshe and Avraham, we have to define the concepts of speech. Words don't fully express the full meaning of what the soul wants to express. Words are limiting to the speaker. [לשון הרב זצ"ל - כשאני מתאווה להגיד מילה, כבר ירדה הרוחניות העליונה מחביון עוזה]. The listener also doesn't fully comprehend what the speaker is saying. Whenever you hear something, it goes into YOUR world and history and perspective and understanding. That is profoundly different than the world of the person who is talking to you. Example: When I talk about Shabbos, I talk about MY Shabbos experience. The food that we have in MY house [4 different kugels, 9 different dips, 6 types of challah, five choices for the main course, 11 desserts. Just joking😀] the shul I DAVEN in, MY seder hayom on Shabbos etc. etc. and the way I process all of these experiences. Your Shabbos is different both externally and internally. So I say "Shabbos" meaning MY Shabbos and the listener is thinking HIS Shabbos. That is one example but EVERYTHING we say is interpreted by the listener to his conceptual world. So there is a double limitation - both with regard to the speaker and the listener.

Seeing is all encompassing. One glance and the person sees it all. A picture says not "a thousand words" but a hundred thousand words, which even after all the words does not fully explain what was actually seen. Three times a year we have a Mitzva to go to the Beis Hamikdash, to see and be seen [חגיגה ב]. "שלש פעמים בשנה יראה כל זכורך את פני האדון השם". The Mitzva is accomplished in a second. One seeing and you did it!! 

Avraham Avinu merited this ראייה. The seeing at the beginning of the Parsha is then limited afterwards to speech and listening. At the end of the Parsha at the Akeida, there is again a ראייה but it relates to the FUTURE. ה' יראה אשר יאמר היום בהר השם יראה. In the future Hashem will see and be seen [כביכול] on this mountain. This ראייה was a test. והשם ניסה את אברהם. Hashem tested Avraham. The word נסיון also means to LIFT UP as in נתת ליראיך נס להתנוסס - I have given those who fear you a test in order to be elevated [a "נס" is a banner or mast].

Moshe saw his prophecy באספקלריה מאירה [a clear looking glass] and due to his clear vision did not merit to "see" Hashem b/c "כי לא יראני האדם וחי" - You can't clearly see Hashem and live. Yeshayahu saw באספקלריה שאינה מאירה [an unclear looking glass] and thus merited to "see" Hashem. "ואראה את אדני ישב על כסא רם ונשא" [See Yevamos 49b]. Since his vision was blurred, he was able to get a glimpse of the Divine. Sort of like [להבדיל] being able to look at the sun if you are wearing sunglasses. 

There is a Machlokes whether Prophecy is on the same level as seeing angels [Rambam] or whether Prophecy is a higher level than seeing angels [see Ramban 18-1]. Avraham first prophesied and then saw angels. First "וירא אליו השם" and then "וישא עיניו וירא והנה שלשה אנשים". However we can't compare this revelation to that of Moshe was was the greatest Navi ever. "ולא קם בישראל כמשה". Yet, the Medrash says that when Moshe Rabbeinu went to Shomayim, the angels wanted to harm him so Hashem made Moshe look like Avraham and told the angels "Are you not embarrassed from him? You went down and ate in his house". In other words, Moshe was masked with Avraham's face, meaning, that his level of Prophecy אספקלריה מאירה needed Avraham's level of Prophecy. It emerges that Avraham's level of Prophecy, behind which shines the image of Moshe, is higher than אספקלריא שאינה מאירה and below Moshe's level of אספקלריא מאירה but in between.

Avraham merits ראייה - seeing, which is then contracted to speech and that is the mask of Avraham's face that Moshe merited when he went to Shomayim and had to prove to the angels that only we are connected to the world of action [see Shabbos 89a]. Moshe himself first merited many revelations of Hashem speaking to him and then asked to see Him "הראני נא". 

The seeing of Avraham was contracted to speech that told him that Hashem was gong to destroy Sodom. The Medrash says that Hashem felt that he had to tell Avraham about His plans b/c he promised the Land to Avraham and now he was going to overturn Sodom. The reason Sodom was destroyed was b/c of the cry of the young girl:  There was a young woman who would take bread out to the poor people in a pitcher so the people of Sodom would not see it. The matter was revealed, and they smeared her with honey and positioned her on the wall of the city, and the hornets came and consumed her. And that is the meaning of that which is written: “And the Lord said: Because the cry of Sodom and Gomorrah is great [rabba]” (Genesis 18:20). And Rav Yehuda says that Rav says: Rabba is an allusion to the matter of the young woman [riva] who was killed for her act of kindness. It is due to that sin that the fate of the people of Sodom was sealed.

Why was Sodom's punishment sealed b/c of the way they treated the young girl who had committed the "sin" of giving Tzdaka? Chesed is not one of the Sheva Mitzvos Bnei Noach??

If they just wouldn't have been people of Tzdaka and Chesed then maybe they would have been spared. But since they made a philosophy of it and it led to murder - that was too much and sealed their fate. 

Why did the angel whose job it was to overturn Sodom come so early?? First, the angels whose job it was to heal Avraham and tell about the birth of Yitzchak [see Rashi 18-2] should have done their jobs and then afterwards the angel whose job was to overturn Sodom could have come. Why did the "Sodom angel" come so early?? 

In order to fully appreciate the extent of their evil, the angel whose job it was to destroy Sodom came to visit Avraham and experience his Chesed. When he saw how a person can reach such heights of Chesed and then compared that with the ignominious behavior of Sodom, he was then motivated to destroy it. 

The reason Hashem first told Avraham that He was planning to destroy Sodom was b/c Avraham thought that they had at least ten men of Chesed who would save them [תנחומא ז]. Only when he realized that they didn't even have a "minyan", he relented and "allowed" Hashem to carry out His plan.

The Parsha starts with the revelation of the infinite to the eyes of Avraham. וירא אליו השם. But his tremendous level did not yet spread to the world so it was then contracted to a speech of מדת הדין when it says "ויאמר ה' זעקת סדום ועמורה כי רבה" - being informed of the coming destruction of Sodom. At the end of the Parsha at the Akeida there is a revelation relating to the future "ויקרא אברהם שם המקום ההוא ה' יראה אשר יאמר היום בהר ה' יראה" [see Rashi "שיאמרו לימי דורות"]. When there is a revelation of infinite ראייה [as expressed in the Mitzva of Aliyah LaRegel], there is no longer a contraction of speech with Avraham. The shining face of the ירא-רואה-נראה [one who fears sees and is seen - see חגיגה ב] is transformed into a נס להתנוסס - a high flying mast/banner [see בראשית רבה נה א] for the whole world to see. 

[עפ"י דברי מרן הרב זצ"ל סעודה שלישית תרצ"ב]