The first Posuk of our Parsha states that Hashem appeared to Avraham in Elonei Mamrei while he sat at the entrance of his tent in the heat of the day (בראשית י"ח:א'). This Posuk certainly attests to the greatness of Avraham when saying that Hashem Himself came to speak to him. Yet in the very next Posuk, we read that Avraham saw some travelers, and he ran out to greet them. Doesn't it seem strange that Avraham, while still in Hashem's presence, would excuse himself to go and greet these travelers?
Rav Shimshon Refael Hirsch explains that this shows how important the Mitzvah of Hachnasas Orchim was to Avraham. Not only was he waiting for any potential travelers in front of his tent, in the heat of the day, while he was still recovering from his Bris Milah, but he also felt the need to excuse himself from Hashem's presence in order to greet these travelers, and to invite them into his tent.
But Avraham didn't bring people into his tent just in order to be hospitable. Rather, he brought people into his tent in order to teach them. He wanted to be "Mekarev" them and bring them closer to an understanding of his belief in Hashem. He wanted to influence them, and he believed that the best way to do this was by being a good host.
We must realize how important the Mitzvah of Hachnasas Orchim is today. There are many Jews in the world who desperately need to be taught, who need someone like Avraham to be Mekarev them. To fulfill the Mitzvos of both Hachnasas Orchim and Kiruv Rechokim, our houses should be open like Avraham's house was open, reaching out to all who want to know more.