"You cannot serve God and mammon" sounds strange when the older word is left untranslated. The term refers to wealth or possessions. Jesus' forceful wording makes wealth act like a master demanding service.
The saying therefore concerns more than whether a person happens to possess money. It asks whom the person ultimately serves.
The word traditionally rendered "Mammon" derives from an Aramaic term associated with wealth or property. In the saying, Jesus places God and wealth in parallel positions as masters. The rhetoric personifies wealth without requiring the conclusion that Mammon was the proper name of a specific deity in this passage.
In a household world where service to a master implied allegiance and obligation, the metaphor is readily understandable: two competing masters cannot both possess a servant's undivided loyalty.
Translating the unfamiliar word restores the practical force of the saying. Jesus is not warning only against an exotic false god called Mammon; ordinary wealth itself can become a claimant upon allegiance.
The master-servant image makes the teaching about loyalty rather than merely quantity. The question is not simply how much a person owns, but whether possessions have acquired the authority of a master.