No, it's not a teaching certificate or a graduate degree, or even a college degree. In the words of John Holt, one of the modern homeschooling movement's founders, this is what people need in order to teach their children:
"First of all, they have to like them, enjoy their company, their physical presence, their energy, foolishness, and passion. They have to enjoy all their talk and questions, and enjoy equally trying to answer those questions. They have to think of their children as friends, indeed very close friends, have to feel happier when they are near and miss them when they are away. They have to trust them as people, respect their fragile dignity, treat them with courtesy, take them seriously. They have to feel in their own hearts some of their children's wonder, curiosity and excitement about the world. And they have to have enough confidence in themselves, skepticism about experts, and willingness to be different from most people, to take on themselves the responsibility for their children's learning" (from Teach Your Own).
I don't think I fit all of these requirements very well when I first started homeschooling (at least not for the majority of the time), but I've found myself increasingly enjoying the presence of my children as we've spent more time living and learning together.