“A Christian is a perfectly free lord of all, subject to none. A Christian is a perfectly dutiful servant of all, subject to all.” What does this mean?

Luther begins this treatise with the paradoxical statement: “A Christian is a perfectly free lord of all, subject to none. A Christian is a perfectly dutiful servant of all, subject to all.” What does this mean? What is Christian liberty? What is Christian bondage? the link is https://www.ccel.org/ccel/luther/christianliberty