Pope Francis will canonize Archbishop Óscar Arnulfo Romero, the Salvadoran champion of poor people who was assassinated as he said Mass in 1980, in a ceremony at the Vatican on Sunday.

The archbishop was one of the most prominent church leaders in Latin America when he was killed at the age of 62. He condemned injustice and spoke out against political repression amid an intensifying war between leftist rebels and government and right-wing forces in El Salvador.

Archbishop Romero had received several threats before a sniper fired a single shot, killing him on March 24, 1980. His funeral, which was attended by huge crowds, also came under attack by snipers. As many as 40 people were killed.

The previous month, Archbishop Romero had angered government supporters by asking President Jimmy Carter to halt military aid to the country. (When Ronald Reagan took office in 1981, he quickly increased such aid.) And in his Sunday sermons, Archbishop Romero insisted on nonviolence as he warned that El Salvador faced full-scale civil war without profound societal changes.