German Catholics are embarking on what is being called a “synodal journey,” but it promises to be a potentially rocky one, focusing on subjects the church usually avoids.

"I am spiritually close to the victims of the episodes of violence that these days have bloodied Texas, California and Ohio, in the United States, affecting defenseless people," Pope Francis said.

The 5,000-word letter focuses on four themes: pain, gratitude, encouragement and praise.

All people of goodwill have a duty to help vulnerable women and other victims of human trafficking escape from forced sexual slavery, the pope wrote in a prologue of a new book on the suffering of women forced into prostitution.

The pope's appeal came after what authorities have called the worst shipwreck on the Mediterranean this year.