It can be discouraging to discover that some of our favorite quotes attributed to some of our favorite theologians were not actually said by them. One example that comes to mind is Luther's statement:
If I profess with the loudest voice and clearest exposition every portion of the Word of God except precisely that little point which the world and the devil are at that moment attacking, I am not confessing Christ, however boldly I may be professing him. Where the battle rages there the loyalty of the soldier is proved; and to be steady on all the battle front besides, is mere flight and disgrace if he flinches at that point.
Except he probably didn't say or write that.
The most-abused apocryphal-theological quote is probably St. Francis of Assis's "Preach the gospel; use words if necessary"---as Mark Galli points out, the quote is fine, other than being unbiblical and not something that Francis said, believed, or practiced!
It can be enjoyable and edifying to track down the originals, which are often fairly close and which have also been uttered by less famous people---as is the case with Bunyan's "Run, John, Run" poem or Luther's statement about the church standing or falling on the doctrine of justification or the original form of the TULIP acronym.
At the Mere Orthodoxy blog, they look today at the origin of the saying, ""You do not have a soul. You are a soul. You have a body." It turns out this actually comes from the pen of George MacDonald, not C. S. Lewis.
At its worst, this quote could be used in defense of what Randy Alcorn has called Christoplatonism, with its suspicion of the body. However, I think the quote could be defensible, insofar as the soul can survive without the body (in the intermediate state which longs for the consummation and the reunion of soul and body), but the body cannot survive without the soul.
Matt Anderson has a good nuanced response to all of this:
We should be careful not to simply be reactionary against uncareful statements like the above. Theology is ever in danger of reductionism, and it's ever possible that our own contemporary reaction against the concept of the soul is too deflationary an account of human persons.
That said, out of context-which is how the legions of people who pass it around Facebook and Twitter generally see it-the quote really does express a stunted vision of the human person in light of the resurrection. My own intuition is to say something along the lines of, "You are a body. But you're a soul too. And your human flourishing is contingent upon being a soul-bodied thing."