No one can serve two masters; for either he will hate the one and love the other, or else he will be loyal to the one and despise the other. You cannot serve God and mammon. Matthew 6:24
Very often, we assume that those who are not serving God are directly serving the devil. But Jesus does not seem to think so. According to Him, men can either serve God or mammon. The Oxford Advanced Learners Dictionary definition of mammon is, "a way of talking about money and wealth when it has become the most important thing in somebody's life and as important as a god." I think that this is what Jesus was trying to warn us against, a situation where our desire for money supersedes our affection for God. When this happens, money replaces God, and then becomes God to us. (This does not deny the importance of money to us).
This is a danger that we all face when boundaries are not set in our lives. No one in his right sense will deliberately opt to serve satan, yet, unknown to many, in living for money, they are indirectly serving satan. Remember what the devil told Jesus in the wilderness, "If You bow down and worship me, I will give all of these things to You." Jesus constantly warns us against the deceitfulness of riches, not because money in itself is bad, but because if we are not careful, we can flip from a healthy disposition to money into the love of money.
Jesus began this teaching, warning against laying up treasures on earth for ourselves. When money ceases to be a means of meeting basic needs in one's life, and helping others in need, it begins to take a role in one's life that can harm his relationship with God. This is when it becomes mammon, and at such times, it is seen as something by which one can find satisfaction in life. This way, money takes the role that only God can successfully play in one's life. The wisdom of God instructs us that we cannot serve God and mammon together; it is either God or mammon.
Toni