The Phrase
"No man can serve two masters" (Matthew 6:24, KJV) - the impossibility of complete loyalty to competing authorities or interests. Jesus's saying about God and Mammon gave English one of its most precise formulas for the problem of divided allegiance and conflict of interest.
Biblical Origin
Matthew 6:24: "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. Ye cannot serve God and mammon." The context is the Sermon on the Mount's teaching on anxiety and provision - the passage immediately preceding the famous "consider the lilies" (6:25-34). The specific application is economic: "mammon" (māmōnā in Aramaic) denotes wealth, possessions, or the spirit of material security.
The logic is psychological and absolute: Jesus does not say it is difficult to serve two masters but that it is impossible. The divided allegiance will not remain genuinely divided - it will resolve, under pressure, into preference for one and neglect of the other. The person who tries to serve both God and wealth will ultimately discover, at the decisive moment, that they have been serving wealth.
Luke 16:13 uses the same saying in a different context, connecting it to the Parable of the Dishonest Manager: the manager who secures his future by using his master's wealth has made a clear choice about who he ultimately serves. Jesus's application is not that wealth is evil but that it cannot occupy the position of ultimate master - that position is already filled.
Semantic Drift
"You cannot serve two masters" entered English as a statement about the impossibility of dual loyalty when loyalties fundamentally conflict. It appears in business ethics (a lawyer cannot simultaneously serve two opposing clients), in political science (an official who serves both the government and a private interest has a conflict of interest that will eventually require choosing), and in personal moral discourse (the person who tries to be faithful to two incompatible commitments will ultimately fail both).
The specific application to God and Mammon has largely been replaced by the general principle, though the phrase's biblical origin is more widely recognized than many other idioms in this category. The specific word "Mammon" - which appears in English as a near-synonym for wealth itself - keeps the theological origin partly visible.
Cultural Presence
The phrase appears in legal and ethical codes, in political philosophy, in literature from Shakespeare onward, and in everyday speech. Its precision - naming not just the difficulty of divided loyalty but its structural impossibility - makes it more useful than vaguer alternatives. The phrase does not say divided loyalty is morally wrong; it says it is practically impossible, which is a stronger claim and a more useful one for the situations in which it is typically invoked.