Genesis 18 gives the account of Abraham's most intimate encounter with the Lord. In this chapter, the Lord appears to Abraham in a human form, stays with Abraham for a long time and even eats a meal that Abraham prepares for Him. This is fellowship on a very human level.
Before we read this chapter together, my concept of the Lord being with us on the "human level” was that the Lord understood and sympathized with our human condition. This is true, but in Genesis 18 it's the other way around! It's the Lord who becomes understandable and knowable. It's His heart that's being revealed, and He's the One who's needs are being met.
Abraham is "selfless" in genesis 18. He only thinks about the Lord's need and also Lot's need. After going through this chapter, the Lord made me aware that my conversations with Him are full myself - my thoughts, my opinions, my needs. In Genesis 18 Abraham didn't interject anything from himself and he didn't ask anything for himself. He only took care of the Lord. His whole being was concentrated on satisfying the Lord. When I try this I find out how impure and full of myself I am in my interaction with the Lord.
I think the Lord's answer to me was that Abraham could be that way for the Lord, so focused on the Lord Himself, because he was circumcised in Genesis 17. Before his circumcision, he would always bring his need to the Lord. "Lord, I don't have a seed. but I have Eliezer" (Genesis 15:2), and "how will I know that you'll give the land to me?" (Genesis 15:8), and "oh, that Ishmael might live before you!" (Genesis 17:8). In genesis 17 God makes it clear that He is the one who is going to take care of Abraham, and that God is fully aware of what he needs. Abraham's circumcision not only meant that Abraham wasn't going to try to meet his needs by his efforts, but that he so fully understood that the Lord was going to take care of it that he didn't even need to think about it anymore. The next person who brought up Abraham's need was not Abraham but God Himself in genesis 18: "Sarah shall have a son at the appointed time, the time of life." This brought out absolutely no reaction from Abraham. Abraham was dead to it. So I think the circumcision in chapter 17 really wasn't just superficial, the Lord touched something deep in Abraham's being.