No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. You cannot serve God and mammon.
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Theophylact of Ochrid
What He means is this: no man can serve two lords who command things that are opposed to each other. Such lords are God and mammon. We make the devil our lord when we make the belly our god. But by nature and in truth God is the Lord, and mammon is unrighteousness.
Do you see that it is not possible for a rich man and unrighteous man to serve God? His love of money drives him away from God.