The most important and necessary expression of freedom is to give your thought a specific direction.
Work toward the purification of your thoughts. Without bad thoughts you will be incapable of bad deeds.
Everything is in heaven's power, except for our choice of whether to serve God or ourselves.
We cannot prevent birds from flying over our heads, but we can keep them from making nests on top of our heads. Similarly, bad thoughts sometimes appear in our mind, but we can choose whether we allow them to live there, to create a nest for themselves, and to breed evil deeds.
— Martin Luther
It is a sin, not only to do bad things, but even to think about bad things.