The notion of a city on a hill that "cannot be hidden" originates, of course, with Jesus in his sermon on the mount. In our moment, we are more familiar with Ronald Reagan's reappropriation of this notion to America, as a beacon of brotherhood to the world.
But the notion was first applied to America in 1630 by John Winthrop, in his famous speech to the new puritan settlers in Boston. In that speech he suggested the context and meaning of the beacon: that God had created the divide between rich and poor, because He is "more honored in dispensing his gifts to man by man, than if he did it by his own immediate hands."
Historically, American exceptionalism may be better understood in terms of this virtue of generosity, rather than isolation.