The fifteenth chapter of B'reishit is the famous Berit Bein HaBetarim. After G' tells Avraham - for the third time - that his descendants will inherit the land Avraham asks G' how he can be sure of this.
ח וַיֹּאמַ֑ר אֲדֹנָ֣י יְהוִ֔ה בַּמָּ֥ה אֵדַ֖ע כִּ֥י אִֽירָשֶֽׁנָּה׃
Before discussing G's response I would like to take up the following question: what was the source of Avraham's uncertainty? Many - perhaps most - commentaries (for instance: Rashi, Ramban, S'forno) explain that he was concerned that his descendants would not necessarily be worthy of such an inheritance (i.e. they might sin) - I do not wish to depart from their model only to add an additional element (which could be viewed as an explanation of what sin (or unworthiness) Avraham felt his descendants might/would be guilty of).
Avraham was a revolutionary. He lived in a society which only knew idolatry. Idolatry was the bedrock of civilization. How so? The idolater believes in multiple deities - each one with its own special powers. By currying favor with his personal deity (or deities) the idolater hopes that his own desires will be fulfilled. There is no grand scheme of creation that he must submit himself to - anything and everything is possible if the deity so desires. The idolater's gods are a tool of his own will. In other words, man is supreme and the gods are useful. The foundation of idolatry is the belief in the supremacy of man. Man's will is immutable - everything else must fall in line, even his gods. That it is the right of man to have power and assert it is the unwritten principle that all states are founded upon. Can there be a state without Malchut Adam, the rule/kingship of man?
Avraham's revolution was not just a simple rejection of idols it was a rejection of the idea of man's supremacy, his claim to power. Malchut Adam is nothing, an illusion - only G' has true Malchut, rule/kingship. Unfortunately, the most likely outcome of Avraham's revolution (if expanded to a large popular movement as opposed to a small tight-knit family unit) is anarchy - after all, he suggested nothing less than the complete rejection of human power. How many individuals would be able to fill this vacuum through submission to G's laws? If the core tenets of idolatry are the foundation of the state how could Avraham's descendants be expected to found a state built on the credo of Malchut Hashem, G's rule/kingship. Nothing less than this belief could make Avraham's descendants worthy of being his inheritors.
I believe this might have been the source of Avraham's uncertainty about his descendants. He could not fathom how an entire state could naturally emerge that could maintain his unique philosophy. It is clear from G's response that indeed that state would not come about naturally.