Historical Protestantism is based on the conviction that God's Word is the final, infallible authority. Liberalism, on the other hand, holds to the modernist ideal that individual experience is the arbiter or truth, and that the only worthy things are that which "help" the individual man.
This leads to everyone doing what is right in their own eyes, resulting in spiritual death.
True soul liberty, on the other hand, brings freedom and spiritual growth. It is found not be doing what "feels right", but by holding solely to the Word and not being bound by the extra-Biblical traditions of men.