Because he denies or contradicts essential Christian doctrine. He takes faith, for example, and he makes it a force. He teaches that words are the containers of the force, and that through the force of faith you can create your own reality. Biblical faith is not the kind of faith Copeland espouses. While he says faith is a tangible force, biblical faith is a channel of living trust between a man and his creator.
Biblical faith is only as good as the object in whom it is placed.
And all faith is subsumed under, or subordinate to and dependent upon, the sovereignty of God. A very important point to make here is that the Faith teachers use Christian terminology and pour their own meanings into those words, which is something all the cults do. They take Christian terminology and pour cultic meanings into the words. Look at Copeland's view of man as an example. He says Adam was an exact duplicate of God when he was created.