Matthew 6:22-24

The eye is the lamp of the body. So if your eye is sound, your entire body will be full of light. But if your eye is unsound, your whole body will be full of darkness. If then the very light in you [your conscience] is darkened, how dense is that darkness! No one can serve two masters; for either he will hate the one and love the other, or he will stand by and be devoted to the one and despise and be against the other. You cannot serve God and mammon (deceitful riches, money, possessions, or whatever is trusted in).