Celebrating Mass with poor people in the splendor of St. Peter’s Basilica, Pope Francis on Sunday denounced those who dismiss poverty as “not my business” and defined indifference to the needy as a “great sin.”

After Mass, the pope lunched with some 1,500 poor people from Italy, Poland, France and elsewhere as the Catholic Church marked its first World Day of the Poor, an event created by Francis to draw attention to those living on the margins of society.

“To do no wrong is not enough,” Francis declared in his homily.

Not using our lives to help others is a “great sin where the poor are concerned,” Francis said. “Here it has a specific name: indifference. It is when we say, ‘That doesn’t concern me. It’s not my business, it’s society’s problem.’”