What was the counsel that Mamre gave to Avrohom?
(18,1) “And Hashem appeared to him in the plains of Mamre."
Rashi explained that since Mamre counseled him to do the circumcision, Hashem appeared to Avrohom in Mamre’s territory. But what was his counsel?
Behold, Avrohom Avinu made every effort to bring converts under the wings of the Shechinah (Divine Presence), without any thoughts of reward. And Hashem blessed his efforts with success, as it says in Bereishis 12:5 “and the souls which they had made in Charan”, which the Targum Onkelos explains means the "souls which they had caused to be subjected to the Torah". The reason he was successful was because the false beliefs of the nations in the days of Avrohom weighed very heavily upon them, because serving their gods required them even to burn their sons and daughters in the fire.
But Avrohom prohesised to them in the name of Hashem to serve the Unique One, and this service required only their faith in Hashem and the observance of seven agreeable mitzvos - not to murder, not to commit adultery, not to steal, not to eat the limb of a live animal, and not to graft one species of tree onto another. Therefore, he merited that his words were heard.
However, now Hashem had requested from him at the age of ninety-nine years to cut off his foreskin, and to do the same to all males when they were eight days old, and Avrohom was afraid that perhaps because of this people would no longer follow him and convert, and therefore it was more acceptable to Avrohom to lose some of his great reward, in favor of continuing to gather souls who will serve Hashem. Therefore, he took counsel with his friends Aner, Eshkol and Mamre, and two of them agreed with him, but Mamre counseled him not to go against the command which he had received from the mouth of Hashem.
According to this we can explain that which is written in the Yalkut on the parsha of the Akeidah (the binding of Yizchok), which says that when Avrohom was going to sacrifice Yitzchok the Satan met him on the way, and said to him, "Old man, to where are you going; is it not to sacrifice your son? He continued, quoting the words of Eliphaz (Iyov 4:2), “Because He tested you with one thing, should you weary? Who can withhold words? Behold, you have chastised many, and you have strengthened weak hands. Your words would pick up the stumbler, and buckling kness you would strengthen. But now when it comes to you, you weary; it touches you and you are frightened”. Avrohom responded: Nevertheless, I am going. Said the Satan: Remember now, who ever perished that was innocent, and where were the upright destroyed? Avrohom replied: With innocence I am going.
The explanation of this dialogue, is that the Satan said to Avrohom: See what you are losing because of this Akeidah. Until now, in the other nations the people were stumbling (because of their cruel practises), but “your words” - that one does not serve Hashem this way, “would pick up the stumbler, and buckling knees” of other nations “you would strengthen”. “But now when it comes to you” that you must sacrifice your only son, “you are weary”. “It touches you” that you are to do what the other nations are doing, “and you are frightened”. Avrohom replied: Even so, the command of the Creator, Blessed be He, is upon me. The Satan responded: Hashem did not speak to you. This is what you heard, “the innocent have perished, the upright have been destroyed”. Avrohom replied: I am not afraid of this, because with innocence I am going, and I will not question, G-d forbid, the command of Hashem.