"What is this, Lucilius, that draws us elsewhere than where we are trying to go, even compelling us to go headlong toward the very place from whence we desire to flee? What is this thing that wrestles with our spirit and prevents us from willing anything singularly? We fluctuate between intentions. We never will anything freely, never absolutely, never with permanence!"
-Seneca's Moral Epistle LII
Ah. Here is the question that every person should be asking. What is this thing? You don't need St. Paul to tell you that you are a house divided against yourself. Even the non-Christian has a will to do good, live honorably, and be at peace with neighbors divine and human. Well then, why isn't the world filled with good, honorable, peaceful persons? Well, there's this thing that wrestles with us.
It is as plain as the nose on your face, and yet it seems like a fantastic insight when someone verbalizes it: there is this thing. It thwarts our intentions and compels us to do the very opposite of what we desire. You don't need a theological diploma to discuss this; if you are talking to a human being, this is familiar territory.
This is a great place to begin a discussion with someone who is reasonable but not necessarily sympathetic to Christianity. This is common experience. Ask the question that Seneca asks: What is this thing? Why can't we fulfill our good intentions? Why do we turn aside from good and even do the very things we hate ourselves for doing? Don't ask rhetorically! Wait for an answer.
Any answer to the question is going to be a mess, because here is the great "black hole" created by sin. What is it? I don't know. Pat answers are not only silly, but insulting to any rational person. "Sin is anything contrary to God's will." Really? I don't think that will get you very far with a thinking person, since now you end up with a god who can't effect His own purpose. "Did God create this thing?" "Is it a thing at all, or maybe an un-thing?"
Whatever it is (or isn't), every human being has first-hand knowledge about it. We may not be able to describe it very well, but we KNOW it intimately. That's what makes it such a great topic for discussion - everybody really IS an expert on this matter.
While we may not be able to define this tendency or evil or... well, whatever it is or isn't, one thing is obvious even to pagan Seneca: it is the nature of our battle with this thing that we can't escape it - it leaves us hamstrung by its very nature. He goes on to write in his epistle:
"Stultitia," inquis, "est, cui nihil constat, nihil diu placet." Sed quomodo nos aut quando ab illa revellemus? Nemo per se satis valet ut emergat; oportet manum aliquis porrigat, aliquis educat.
"'Is it not the fool,' you say, 'that is never constant, never satisfied for very long?' Rightly so, but how will we extricate ourselves when we realize that we are such fools? No man is strong enough in himself to escape; he needs someone else to reach out his hand and pull him out."
Discussing the existential struggle we endure within ourselves leads us inexorably to this conclusion: we need someone outside of ourselves to get us out of this mess. This wrestling isn't natural (a very important conclusion for those familiar with Stoic teaching), and there must be an end to it. There is, in fact, an end to the struggle. Someone else has reached out his hand to pull us out of the struggle - not today, but on the Last Day. It requires faith in the faithfulness of God in Christ to fulfill His promise of deliverance through death and resurrection. Far from being an irrational faith, it is a rationally necessary faith if an end to the struggle is ever to be found!
"So I find this law at work: When I want to do good, evil is right there with me. What a wretched man I am! Who will rescue me from this body of death? Thanks be to God—through Jesus Christ our Lord! And if the Spirit of him who raised Jesus from the dead is living in you, he who raised Christ from the dead will also give life to your mortal bodies through his Spirit, who lives in you."
-St. Paul to the Romans (Chapters 7 and 8, ESV)
Seneca's writings are available from Harvard University Press' Loeb Classical Library: http://www.hup.harvard.edu/loeb/