Hashem appeared to him in the plains of Mamre while he was sitting at the entrance of the tent in the heat of the day: (Bereishis 18:1)
Avrohom Avienu sat at the door of the tent. We may expound on this that not only was Avrohom Avienu awaiting to fulfill the Mitzvoh [commandment] of Hachnosos Orchim (inviting and hosting guests), but so too was his Kedusha [Holiness] a beacon for his home within and outside.
This brings to attention the following story which happened to my Great Grandfather Reb Hersh Hagodol of Shidliv ZY"A – Talmid Chover HaChozeh Hakodosh MeiLublin ZY"A.
The Chavrayoh Kadishoh [holy group of Tzadikim] was once travelling, at nightfall they arrived at a town which only had a barn to sleep in. The problem was that it did not have a Mezuzoh on the door. The great group concluded, that since Reb Hershel is a "Shomer HaBris – an adherent of the Holiness of Bris", he shall stand at the door all night to protect the inhabitants of any Mazikim (demons). Another version is, that it was on the way to the Usteloh Chasunoh, upon their arrival, one of the Tzadikim asked them about their lodgings, he questioned them about the barn, to which they answered, “Reb Hershele was the Mezuzoh.”
From Avrohom Avienu and the above Tzadikim, ZY"A, we must learn the following, amongst the many things. Firstly, Avrohom Avienu was purely looking to do the Mitzvohs of Hashem [G-D]. Secondly, at the same time he was standing guard at his door that no bad influences enter. As the first thing he told his guests was to wash their feet, as no Avodoh zoroh [idol worship] entered his house.
We also see the following two aspects, it was a Mitzvas Asei and Lo Sasei (to adhere and to abstain). We also see the greatness of the Mezuzoh, which watches within and protects from the influences of outside.
