'Mammon' is an ancient word that means 'wealth' or 'riches,' and is used in the Bible in a negative way, denoting worldly wealth as opposed to spiritual wealth. 'Riches' in the Bible represent knowledge and understanding of spiritual things -- good things to have. But such knowledge is not restricted to good people; it's possible to know and understand a great deal about spiritual things and not use it, or to have it and use it toward selfish ends. This is the kind of behavior that 'mammon' describes.