My missus got an online TEFL to work at a uni here in China. She didn't have a teacher's degree so it was of some use as far as a certificate went. The real work starts when you find yourself at a school in front of kids (even students at unis are kids to an old fogee like me). At that point, you are either a teacher or you aren't. Important to realize, that a teacher does make mistakes AND learns from these mistakes knowing that each new class, year, school will provide you with all kinds of opportunities to continue learning, to continue making new and creative mistakes.