The annual pilgrimage of Arbaeen is one of the most significant events in the Shiite religious calendar, when millions of Shiites from Iraq and around the world come together to travel on foot to the holy city of Karbala.

Arbaeen moves not only Shiites but also members of Iraq’s other religious communities to action, such as Christian Samir Mate, a 34-year-old Christian resident of Babylon (Babil) Governorate....