Full Idea
Schiller constantly speaks of spiritual freedom: freedom of reason, the kingdom of freedom, our free self, inner freedom, freedom of mind, moral freedom, the free intelligence - a very favourite phrase - holy freedom, the impregnable citadel of freedom.
Gist of Idea
Schiller speaks obsessively of freedom throughout his works
Source
report of Friedrich Schiller (works [1794]) by Isaiah Berlin - The Roots of Romanticism
Book Reference
Berlin,Isaiah: 'The Roots of Romanticism' [Pimlico 2000], p.79
A Reaction
Kant's philosophy and his Kingdom of Ends are an obvious source for this, but I trace the sentiment back to 'Freeborn John' Lilburne during the English Civil War. The English, thanks to Voltaire, embodied freedom in the Enlightenment.