Think of someone who has hurt you and ask God for the strength to forgive that person, Pope Francis told the crowd gathered in St. Peter’s Square on Sunday.

Speaking from a window of the Vatican’s Apostolic Palace on Sept. 17, the pope underlined that forgiveness can heal “the poisons of resentment” and “restore peace to our hearts.”

In his Angelus message, the pope said that forgiving is “not a good deed that we can choose to do or not do” but “a fundamental condition for those who are Christians.”

“Every one of us, in fact, is ‘forgiven,’” he said. “God gave his life for us and in no way can we compensate for his mercy, which he never withdraws from his heart. However, by corresponding to his gratuitousness, that is, by forgiving one another, we can bear witness to him, sowing new life around us.”

“For outside of forgiveness, there is no hope; outside of forgiveness there is no peace.”

The pope compared forgiveness to “oxygen that purifies the air polluted by hatred” and heals the “many diseases of the heart that contaminate society.”

He reflected on Jesus’ response to Peter, who had asked: “Lord, if my brother sins against me, how often must I forgive him? As many as seven times?”

“Jesus answered, ‘I say to you, not seven times but seventy-seven times (Mt 18:21-22).’”

Pope Francis added: “Jesus’ message is clear: God forgives incalculably, exceeding all measure. This is how he is; he acts out of love, and gratuitously. … We cannot repay him but, when we forgive a brother or a sister, we imitate him.”

“May Mary, Mother of Mercy, help us to receive the grace of God and to forgive each other,” he said.

After praying the Angelus prayer in Latin with the crowd, Pope Francis noted that he will travel to Marseille, France, on Friday to attend a meeting of bishops from the Mediterranean region that will have a special focus on the issue of migration.

He said that migration is a “challenge” that must be faced together, adding that the future will only be prosperous if “it is built on fraternity, putting human dignity first … especially for those most in need.”

Pope Francis said that Marseille is called to be “a port of hope” and asked people to pray for his upcoming journey to the French city Sept. 22–23.

This article was originally published on Catholic News Agency.