Pope Francis, who chose his papal name after the 13th-century figure St. Francis of Assisi, known as the patron saint of ecologists, was the fiercest climate and environment advocate in the church’s history.

Celia Deane-Drummond is a theology professor at the University of Oxford and director of a research institute named after the 2015 papal letter. In a piece published the same day Pope Francis’s death was announced, she looked at his influence on the global climate movement.