You should seek out other Christians instead of those Christians you described.
You would also loose your ideas about "most Christians who label self-development as sinful" this way.
So-called Bible-believing, fundamentalist Christians are not the "most Christians".
Educating yourself is self-development (and a "good Christian" should strive to educate himself in such areas as history and languages for understanding the bible and the bible's context), improving yourself to get rid of "sinful" behavior is development, too (like stop being slothful).
Ah, the problems being religious creates, it is truly wonderful.