2 Corinthians 1 and 2 has some interesting notes. I like the point he makes early in the letter about how God comforts us and we can, in turn, comfort others. There's an interesting verse there that says, "For just as the sufferings of Christ flow into our lives, so also through Christ our comfort overflows."
I usually try to avoid a real "us vs. them" mentality when talking about those human beings that do not believe in, cling to, trust in, and rely on Jesus, our God and Savior. It always seems to me like that attitude is a turn off and can lead to pride and condescension (not good things).
But Paul really seems to draw a line in the sand when talking about people "of the world" in making plans and say "Yes, yes" and "No, no" in the same breath. He is obviously using this as instruction to us, that we may not be double-minded and unfaithful in our plans and commitments. And surely there are people that choose not to believe who have built a reputation on honesty and trust -- people that surely you could depend on to mean yes when they say "Yes," but nevertheless Paul refers to this wishy-washy behavior and planning as "worldly."
Food for thought.
