Therefore consider whether the light in you is not darkness.
Luke 11:35 is from Luke's version of Jesus' teachings that Matthew presents as the Sermon on the Mount (see Mt. 6:22). Jesus begins the thought by noting that no one lights a lamp and then puts it in the cellar, or under a bushel, so that the light would accomplish nothing. A light is meant to be shared. So it is with the spiritual truth that he is teaching—the indwelling Christ Presence that is our true identity and our Oneness with God. It's not enough to know it; we have to hold its light high so that it dissolves the darkness and transforms the world around us. And if the spiritual belief we affirm does not create light, but rather contributes to the enveloping darkness, then we have to 'rethink' the way in which we're expressing it. You ask how we know, and I think Jesus explains that elsewhere in the Sermon on the Mount: “by their fruits you will know them.” If our spiritual energies are generating anger, judgment of others, arrogance, then truly our '”light” is contributing to the darkness. We are simply to allow our inner light to shine. We don't decide where that light should go, or who deserves it, or what others have to do or believe in order to receive it. We let the light shine, and the infinite Light and Love of God take it from there. So the key, I think, is in not just congratulating ourselves on our spiritual light, but being very aware of the effect our light is having on the world around us.