I recently finished reading a book (which will remain nameless and author-less, for the purposes of this post) in which the Gospel was explained purely in terms of light and darkness. That is, Jesus was repeatedly described as "the Light" and we are described as in need of rescue because we live in the "darkness." When the book explains the nature of sin, it explains that sin = darkness.
And of course this is perfectly Biblical language. After all, Jesus is the Light that shines into darkness, the true Light which enlightens everyone, and has no darkness in Himself. That Jesus took the darkness in us on Himself on the cross (sorry for the preposition overload) so that we too could walk in the light is a useful and, again, a Biblical way to articulate the Gospel.
Yet, this is a problematic way if it is the only thing we do. Light and darkness are indeed important Biblical ways to articulate key doctrines, but using this language exclusively does what the Bible most emphatically does not do: it robs the Gospel of its moral language. To say that we are in "darkness" without going on to explain the moral depravity of that condition doesn't really convey the whole truth of the human state. We are in darkness, but that darkness is not a mere absence of the light, it is a wickedness that exists in active rebellion against God. John 1 and I John 1 both have to be read along with Romans 1 and the whole law of God. We are not merely ignorant because of the absence of light, we are evil because we have broken God's law.
Only when we understand the fullness of the Gospel in these terms, can we get the full impact of what it means for the Light to shine in the darkness. The substitutionary atonement on the cross is fundamentally a moral and judicial event, where all of my "darkness"--with darkness meaning moral depravity and rebellion against God--is taken by the one who is perfectly Light--with "light" meaning perfectly moral, good, and holy. The language of light and darkness brings a sense of wonder and triumph to the table, but the moral and judicial nature of the Gospel tell us why it is a triumph, and what sort of triumph it is.
Which leaves me in a bit of a bind as far as how to rate this book (theologically, at any rate). Again, it's using perfectly Biblical language. The issue is that it is incomplete. It's as if someone were to try to explain the Gospel using only farming imagery. You could do it, and even be Biblical in your language while doing so, but I think at the end of the day something would be left out. Which isn't to say that one couldn't come to Christ by that means, just that, well, something's incomplete about the explanation of Christianity.
And I see that I'm starting to repeat myself, so I'll cut the post short here. Really, I'm just using this as a chance to help myself think through this issue...