What Is "The Sin of All Sins"?

Our present Pope, as well as many of his predecessors, including Pope Emeritus Benedict XVI, Blessed John Paul II and Paul VI, have lamented the fact that we have "lost the sense of sin". This should be obvious when we take an honest look at ourselves and the world around us.

But what is "the sin of all sins" and why is it eternally vital for each of us to know the answer to this question? 

Because knowing the answer to that question, a wise Trappist author suggests, is to discover the means of recognizing and avoiding sin in the future: