We've been brought out of darkness, into marvelous light! In a spiritual sense, darkness implies ignorance or error, whereas light refers to illumination (or knowledge that enlightens the mind, soul, or conscience). We could also say light is true knowledge of God and spiritual things.
The problem is, the only way to operate in these truths is to BELIEVE these truths. There are truths that you will never hold the power of until you believe. (This is the secret!)