OK, so you have paired sins and virtues, as has been suggested. - e.g. Lust/Chastity. Each of these will have a value and ala Pendragon, they will be linked and the value of the sin/virtue will be reflected as a dice pool. So, for example, you could have Lust (4)/ Chastity (3). When acting lustfully, you roll 4d6, when acting chastely, you roll 3d6.
When you make an action within the game that falls into the realm of a sin or a virtue (Almost all actions can be stuck into one of them. A combat situation is Wrath. A chase scene is Pride. Stealth is Sloth, Social skills are Lust, Endurance style situations are Gluttony, Medical can be Envy (its more like Kindness, the virtue). Still struggling a little for Greed) you roll your dice against a target number. Roll over, win etc.
The trick comes when the GM takes the role of the Tempting Devil. In any action, the GM can offer you extra dice, but will cost you points from your virtue. So, for example, the GM might off you an extra +2d6 on your roll, at a cost of 1 point of virtue. You stand a better chance of succeeding if you move towards the 'dark side'.
I'm also looking at having the difficulty numbers being ... fluid ... so that the more infernal influence there is in an area, the higher the difficulty numbers are - so you may have to be tempted or risk the horrors of probability.
The final arm of this mechanic would be that whenever you are successful tempted 'Hell' gets a point to their total and this would eventually move Hell towards an endgame. And yes, this would get to a point where the agents might well tip the scales and start that endgame.
Now, this is about the vaguest representation I can think of, and as usual my initial mechanics for games tend to rotate around some sort of bluff, barter, game-theoryable mechanic.
I would appreciate your thoughts on whether this is madness!