No man can serve two masters

A man with two employers cannot be faithful to both. Nor can a man support two conflicting causes. The source is Matthew, vi, 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 despite the other. Ye cannot serve God and mammon.’

See also:

• A door must be either shut or open
• You cannot burn the candle at both ends
• You cannot have it both ways
• You cannot have your cake and eat it
• You cannot run with the hare and hunt with the hounds
• You cannot sell the cow and drink the milk