Where did the idea that there are two kinds of Christians, carnal and spiritual, come from? It apparently originated in the mind of a dispensationist, yeah, that's right, a dispensationist. His name? L.S. Chafer.
Two kinds of Christians? Might just as well rewrite Romans 8 - you know, like put a whole lot of qualifying statements in there. For example: Romans 8:14 - For as many as are led by the Spirit of God, these are the sons of God... should read "For as many as are led by the Spirit of God (though some might go on to be carnal, and therefore this verse should not necessarily be used to define all Christians) these are the sons of God".
You get the point.
A great paper on the subject can be found here...