The idea of the inevitability of sin can sweeten some of the most bitter, even cruel verses in the Torah. It's tragic enough that Moshe, without whom there would be no Jewish people to enter the land, is denied entry into the land even after repeated entreaties. But when God begins to tell him, in chapter 31, of the ways that the Jewish people will rebel in his absence, rejecting everything he is trying so hard to pass on to them, it seems to cross the line from tragic into torture. Moshe was obviously anxious about the possibility that this would happen. Did God need to be so blunt?
But God's words are actually a comfort and an antidote for his anxiety. Satan's gravest, most dangerous line of argument is: give up. You're too far gone. There's no use trying. Whatever type of self improvement plan a person will try, from a new diet to a new religious lifestyle, that voice will pop up after that first, or second, or third inevitable mistake. It's the voice which tells us that every sin is of ultimate significance, that if we've fallen once, we'll fall again, so what's the use trying.
What God tells Moshe is just the opposite. Don't worry. The Jewish people will sin. I guarantee it. And the sin will have consequences. But it won't mean the end of the relationship; it's a natural part of the relationship.
The rabbis takes it one step further in the Talmud. Taking the verse "Behold, you will rest with your ancestors, and this nation will rise and stray after the strange gods of the land," they intentionally misread the syntax to yield "Behold you will rest with your ancestors and rise," thus proving the idea of the future resurrection of the dead. Perhaps there is something deeper at play here than just a playful reading. Perhaps it is the inevitability of future sin that suggests that, just as God is eternal, humanity's dance to and from God can have no absolute end, and so even those who exited the dance long ago must at some future point rejoin it. Sin is not just a natural component of human existence; it is the justification for continued human existence.