Francis wrote, "our human family is beset by grave humanitarian crises demanding farsighted and united political responses."

The week of prayer, Pope Francis said, urges Catholics and Lutherans to reconcile and "draw closer to one another anew through conversion."

Pope Francis puts his credibility on the line not only in the foreign policy field, but also in the pastoral field, outgoing U.S. Vatican Ambassador Ken Hackett says.

The remarkable showdown is the latest example Pope Francis clashing with more conservative elements in the Catholic Church.