יהושע, פרק ב׳, פסוק כ׳

The agreement forged between the Israelite spies and Rahab comes with strict boundaries to ensure its success. A clear condition is set, defining exactly when their promise of protection would become void.

The spies warn Rahab that she must keep their arrangement completely hidden. Specifically, they forbid her from sharing the secret sign and signal they had established [מצודת דוד]. This warning serves as an absolute ban on telling anyone that her home is designated as a safe haven [ביאור שטיינזלץ].

The practical reason for this strict condition is a matter of security. The spies worry that if Rahab shares the news of her guaranteed safety with the rest of the city, crowds of people will rush into her home seeking shelter from the upcoming invasion. If she were to let this happen, the spies declare that their agreement would immediately expire. They would be entirely freed from the oath she made them take, and they would no longer have any obligation to spare the lives of those gathered inside [רלב״ג].