milan pope francis sunday call world leaders work favor nuclear disarmament protect human right particularly weak underprivileged people pontiff say need work determination build world without nuclear weapons speak window papal apartment overlook st. peter square cite 2015 encyclical letter laudato si praise remark come day group year nobel peace prize urge nuclear nations adopt u.n. treaty ban atomic weapons rise tensions unite state north korea pope repeatedly warn catastrophic humanitarian environmental effect nuclear devices call third country mediate dispute weekly angelus prayer pope francis add men women world liberty intelligence capacity guide technology limit power service peace true progress speak aboard plane back trip myanmar bangladesh pope suggest world leaders irrational attitude toward nuclear weapons last month appear harden catholic church teach nuclear weapons say countries stockpile even purpose deterrence pope francis strong defender environmental protection also hop upcoming paris summit would adopt efficient decisions contrast climate change pope pray nuclear disarmament