A witness is someone who has seen a thing for himself, not someone who repeats what he was told. You are asked to witness that the awareness of being, the plain sense of I am within you, is the only power there is. No god was formed before that awareness and none will be formed after it, because everything you call a power outside you is something you first accepted inside.
To know and believe here means to choose the inner sense of being over every report your senses bring back. The senses insist the world makes you. The awareness answers that you make the world by what you assume yourself to be. When a person stops arguing from the evidence and rests in I am he, he stops looking for God elsewhere. He has found God as his own consciousness.