Kabbalah Kejadian 18:1: Reshit Chokhmah & Zohar

And here, with the concept of surrender, the humble [person] finds himself with the the Creator - Hashem. Just as He is elevated above all of those elevated, and 'gaah' above all of those who are 'gaim', just as we said with what was written (in Tehillim, chapter 33:1), "ה' מלך גאות לבש." So too [we find humility] with Dovid haMelech (alav haShalom) (in Tehillim, chapter 18:36), "וענותך תרבני," as it is explained in Midrash Tanchuma (ילקוט שמואל רמז קסא) about this passuk. This is what it says. "With my right [hand], I support you," (Tehillim 18:36) -- this is what is written in the Torah, "מימינו אש דת למו" (Devarim 33:2). And, "Your humility is multiplied," as in, who is humble like Hashem? A person says to his friend: Come and beautify the brides, or visit the sick, or bury the dead, or give comfort to the mourners: and he says to him, "I am a 'gadol' and rich; why should I go [and do these]? However, Hashem [does each of these, as it is written in these pasukim]: ויבן את הצלע (Bereishit 22:2), וירא אליו ה' (Devarim 18:1), ויקבור אותו בגי (Devarim 34:6), וירא אלהים אל יעקב (Bereishit 25:9).

Other examples: "ותתן לי מגן ישעך, אל תירא אברם אנכי מגן לך (Bereishit 15:1). וימינך תסעדני, בכבשן האש, במלכים, וברעבון [דכתיב] נאם ה' לאדוני שב לימיני (Tehillim 110:1)." From these, we see that Hashem's humility is vast. We see this also [with Avraham avinu], that he [Avraham] was sitting, and the Shechina was standing, as it says (Bereishit 18:1), "וירא אליו ה' באלוני ממרא והוא יושב פתח האהל."

The answer is that the word “him” here has an inner meaning: it refers to the grade which now spoke with him. Now for the first time “the Lord appeared” to that grade; that is to say, the Voice was revealed, and associated itself with the Speech (dibbur) in conversing with him.