On Non-Christians and the Existence of God

What I’m wrestling with at the moment is why that one thing doesn’t seem so self evident most of the time. Perhaps I am suppressing the truth by my wickedness. Perhaps that’s really the essence of sin—not really believing that and acting accordingly. I guess it is then in my own sinful self-interest to avoid that particular piece of knowledge. I know a couple of my friends are wandering away from the faith at the moment, because they don’t really see why they need God. Knowing that our primary function and greatest good is knowing him is completely opaque to them. Deep down they don’t really believe that they’re all that bad, and for the most part they’re happy in their comfortable middle-class, tertiary educated lives. How do I show them that our rebellion is horrifying, evil, disgusting–not just intellectually credible? And how do I do it with humility and gentleness?