It is said that one of the great qualities of Avraham Avinu is that he had faith in Hashem. His faith was one of the highest levels man can reach. Why then do we find him questioning Hashem? In this week's Parsha, Hashem tells Avraham that he will inherit the land; Avraham then asks how he will know that he will indeed inherit it (בראשית ט"ו':ח'). Does this sound like faith?
Chazal say that he was asking not how he will know, but what he has done to deserve it. The Sefer HaKesav VeHaKabalah quotes from the Tzeidah LaDerech, however, that although it's hard to understand why, because of this question Bnai Yisrael had to eventually go into Golus. To "punish" Avraham, Hashem told him of the forthcoming exile. Other Meforshim, however, do not look at this negatively, but follow the simple interpretation and say that the question was for clarification. The Ramban, for example, says that Avraham wasn't questioning whether Hashem was telling the truth, but was rather worried that the sins of his children might cause Hashem to take away the land from Bnai Yisrael and give it to someone else. The Ralbag disagrees with this, though, saying that since this was a covenant, sins could not cancel it. Others thus say simply that Avraham was asking for the details of the promise, like when, where, how much and so on.
The Tur quotes some who take even the promise of Golus in the positive sense, explaining that Avraham was saying that since he was living among the gentiles and would be forced to make a treaty with them, and the same will happen to his descendants after him, how will he and his descendants ever inherit all of the land? Hashem answered that they'll be sent into Golus for four generations and when they return, they'll have no ties to anyone in the land and will thus be able to evict them without a problem. Finally, others, like the author of the Sefer HaKesav VeHaKabalah himself, say that Avraham wanted to know simply why he deserves this promise.
There is another question that may be raised. If we look at the full Posuk (שם), we find that Avraham said, "Hashem, Hashem, how will I know that I will inherit it?" But the name of Hashem is spelled differently each time. First, it is spelled Aleph-Daled-Nun-Yud (א-ד-נ-י); then, it is spelled Yud-Kay-Vov-Kay (י-ק-ו-ק). The problem is that the Torah later states (שמות ו':ג') that the Avos, included Avraham, did not know the name of Hashem spelled י-ק-ו-ק. Yet here we see that Avraham not only knew that name, but he even used it! Moreover, Hashem, when speaking to Avraham (בראשית שם פסוק ז'), reveals Himself with that same name! Some explain that indeed the Avos did know that name, but they did not comprehend its true meaning. They thus received their prophecy through a different name.
One could, however, suggest another explanation. That name of Hashem is used in connection with the fulfillment of a covenant between Hashem and the Jewish nation, as indicated by Rashi (לשמות שם פסוק ב' בד"ה ויאמר); the growth of the nation is the first part of that covenant. The name is thus also used in connection with the establishment of the covenant, in the Pesukim in our Parsha cited above. Therefore, when Avraham asks how he can know he will inherit the land, he wants to make sure that the promise will be carried out through that special name of Hashem. And as for his question about deserving this gift, Hashem prepared for that question by using that very name, thereby hinting at Avraham's first use of that name. When Avraham returns from battle, he praises Hashem with that very name (בראשית י"ד:כ"ב); that is the first time that name was used in speech. He used it to reject a monetary reward for his part in the battle, saying instead that he trusts that Hashem will make him rich. It is that trust, expressed with the use of the י-ק-ו-ק name, that brought Avraham the reward of getting the land, also expressed with the use of that name.
Finally, Avraham's fear that others would take the land from his descendants is also addressed. Just as those allies of Avraham in the battle, namely, Aner, Eshkol and Mamrei, are taken care of independently (שם פסוק כ"ד), without interfering with Avraham's relationship with Hashem (characterized by that special name of Hashem), so too will other nations be taken care of by Hashem without interfering with the relationship between Avraham's descendants and Hashem. The three major interpretations of Avraham's question now each fit in with the Pesukim, revolving around the use of Hashem's special name.