17. Am I hiding from Avraham what I intend to do (to S'dom)? 18. And Avraham will become a great and mighty nation.....
Rashi explains that Passuk 17 presents an implicit reason for Hashem's decision to tell Avraham about the future of S'dom. As Rashi says, the land was promised to Avraham, and he is Av Hamon Goyim, the father of multitudes of nations; if the residents of his land are going to be destroyed, he should be told. But passuk 18, le'goy gadol, that Avraham is the progenitor of a great nation, doesn't seem to have any relevance to Hashem’s decision to let Avraham know about the fate of S'dom.
Rashi offers two possible explanations of passuk 18. 1. That it was simply a bracha to Avraham; having mentioned Avraham, Hashem gave him a bracha, and it has no particular relevance to the matter at hand; or 2. Avraham is dear to Me, so I will not hide anything from him.
In our case, the episode makes it clear that the logic of tefilla is beyond human comprehension, and we are left on our own to devise what tefilla we think might be most effective. Our choice of words might be inelegant or even primitive, but whatever approach we use, whether it is a request for consideration of the secondary consequences of enforcing judgment, or a prayer for a second chance, or we simply ask for a gift of clemency, the essence of tefilla as taught to Avraham Avinu is the appeal to Hashem's mercy. Avraham Avinu had a hava amina that tefilla would be of no avail where a gzeira was a well-deserved judgment for terrible sins. Hashem told him that even where you can’t come up with a good argument, Tefilla is a powerful tool that can change reality- Middas Hadin to Middas Harachamim, and, ultimately, barren old age to fresh and fruitful youth. Unfortunately, some people read the story and remain with the Hava Amina.
Here is another interesting example. Julian the Apostate, the last non-Christian Roman emperor, planned to allow the Jews to rebuild the Beis Hamikdash, but delayed putting his plan into effect until he finished a military campaign against Iran. He was mortally wounded during that war and died before he could carry out his plan. Julian was named The Apostate by the Christians for his rejection of Christian belief. He wouldn't count for a minyan, but he seems to have been, for a Roman Emperor, a pretty decent person. Julian recorded some thoughts about the Bible, and his comment about the Bris Bein Habesarim is fascinating; he writes that the reason Avraham Avinu cut the animals in half was so he could divine the future by examining their entrails, and that he was told to go outside and look at the stars so that he could foretell the future by his Astrology. How ironic! Although we know that Avraham Avinu was at one time respected for his prognostications (Bava Basra 16b), this episode is, for us, the source for the idea of Ein Mazal L’yisrael (Nedarim 32a and Shabbos 156a), that astrology is utterly insignificant for the servants of Hashem, and poor Julian saw the opposite.