"There is nothing so opposed to the Christian character as over-indulgence."
This seems like an awfully bold claim for our Father Benedict to insert nonchalantly into the Rule. Perhaps he's utilizing hyperbole to emphasize "Our Lord's words, 'See to it that your hearts be not burdened with over-indulgence,'" but what if he isn't? Why would he claim that this behavior holds the highest rank among things that oppose the development of the Christian character?
For an answer to this question I turn to a concept that would have been central to St. Benedict's thinking as a monk in the 6th century. It is an understanding of human formation that comes from the Egyptian desert fathers, and would be popularized in the generation or so after our Father Benedict through St. Gregory the Great. It is the notion of the deadly passions (sins).
Evagrius Ponticus was a late 4th century Egyptian monk and ascetic from whom we learn about eight deadly passions. For Evagrius, each passion leads a person further and further away from what is real, beginning with the most basic needs of survival and moving to the essence of what makes us distinctly human (i.e., the capacity for divine union). His formulation begins, as does every other formulation of the deadly passions, with gluttony, which is the first among the "Passions of Desire." These are ways we misuse our natural impulses, and they also include fornication and love of money. Then come the "Passions of Reaction", which are passions directed against others and include depression, anger, and listlessness (later, accidie). Finally, Evagrius lists two "Passions of Sense of Self", which are fantasies of self: vainglory and pride.
So, back to Chapter 39 and our Father Benedict's claim that "nothing is so opposed to the Christian character as over-indulgence." Over-indulgence, or gluttony, is the gateway passion. It is the most basic misuse of our natural impulses and the easiest. But as such, it has the power to divert our path without us even knowing it and to place us in a prison of the most base, least developed experience of a human being. Resisting and overcoming gluttony, then, is an essential component of a Christian's formation into the likeness of Christ and her first step towards experiencing her true identity hidden in God.