Here is an excellent article by Hank Hanegraaff about the problem with prosperity teacher Joel Osteen's theology:
Behind Osteenian self-affirmations—“I am anointed,” “I am prosperous,” “My God is a ‘supersizing God’”—there lies a darker hue. Behind the smile is a robust emphasis on all that is negative. If you are healthy and wealthy, words created that reality. However, if you find yourself in dire financial straits, contract cancer, or, God forbid, die an early death, your words are the prime suspect. Says Osteen, “We’re going to get exactly what we’re saying. And this can be good or it can be bad” (Discover the Champion in You, May 3, 2004).