I think the greatest sin, the mother of all sins -- not in so much as its apparent severity, but rather the sin from which so many other sins are born -- is ingratitude.
Are you familiar with The Seven Cardinal Sins? Colloquially, many people call them The Seven Deadly Sins. And with good reason.
Can you name them?
I believe each of these sins stems in some way from just one: ingratitude.
ENVY: If you are ungrateful for what you have, then you envy what someone else has, because surely his or hers is better.
GLUTTONY: If you are ungrateful for what you have, then the taste of it is insufficient to satisfy your cravings, which increase in direct proportion to the amount you stuff into your gaping maw.
GREED: If you are ungrateful for what you have, then it's never enough and you yearn avariciously for more.
LUST: If you are ungrateful for what you have, then your eyes and your thoughts wander over to what your libido tells you are greener pastures.
PRIDE: If you are ungrateful for what you have, then what you have not only is too little, but beneath you, because clearly you deserve not only more, but better.
SLOTH: If you are ungrateful for what you have, then you are unwilling to lift even a finger to improve it, because even though it's so awful, it's lulled you into a level of comfort which would be uncomfortably disturbed should you rouse yourself to effort.
WRATH: If you are ungrateful for what you have, then you are stirred to wrath by the insufficiency, ineptitude, or injurious nature of those around you whose disruptions, demands, or delays apparently thwart your brilliantly laid out plan for your life.
See what I mean? Deadly, aren't they? And yet, not so far away for each of us.
But, fear not! For behold, I have a solution. Thankfully, the Catholic Church has gifted us with Seven Cardinal Virtues, which inversely correspond to these horrible vices. Cultivate these and you'll be out of the woods in no time!
Are you ready?
Envy - Kindness
Gluttony - Temperance
Greed - Charity
Lust - Chastity
Pride - Humility
Sloth - Diligence
Wrath - Patience
Cultivate these virtues with due care, and you'll quickly be on the proverbial yellow brick road to sainthood!