In the Parsha this week there were three angels that came to visit Avraham. Rashi says every angel is given one mission, so there was a need to have three different angels. One was to tell Avraham he would have a baby, one was to destroy Sodom, and the third was to cure Avraham from his circumcision. But there were four missions - the fourth was to save Lot. The angel that helped cure Avraham eventually saved Lot. So why weren't four angels sent to visit Avraham?

The Chidushei Ha-Rim gives an interesting explanation. Lot was saved on the merit that he was the ancestor of David. David was the son of a Moabite, and Moabites were prevented from being Jewish because of an incident in the Desert during the Exodus when they did not come out of their camp to greet and feed the Jews. However it was later decided by the Rabbis that this only applied to Moabite men because it is the custom of women not to come out to greet strangers due to modesty. So David was considered a full fledged Jew since his Moabite lineage was matriarchical only.

And how did the custom become clarified that women stayed inside when visitors came? In the very incident that occured right after the three angels came to Avraham, as the verse states, "'Where is Sara your wife (asked the three guests),' and Avraham answered 'Hinei b'ohel - she is inside.'" Until Sarah clarified this custom of modesty, Lot's merit was still in the balance, since the ruling regarding Moabite women hinged ultimately on this very act of Sarah. So because Lot's fate was not decided yet, there were only three angels sent at that time, and the angel that saved Lot had not been sent yet.

- Wellsprings of Torah, Parshat Vayera