The pope quoted the opening prayer for the first Sunday of Advent: "Grant [us] … the resolve to run forth to meet Christ with righteous deeds at his coming."
"Jesus is coming, and the road to meet him is clearly marked: it passes through works of charity," he said.
"Charity is the beating heart of the Christian: just as one cannot live without a heartbeat, so one cannot be a Christian without charity."
Following the Mass, Pope Francis prayed the Angelus from the window of the Vatican's Apostolic Palace with pilgrims gathered in St. Peter's Square.
"Today, the First Sunday of Advent, a new liturgical year begins. In it, the Church marks the passage of time with the celebration of the main events in the life of Jesus and the story of salvation. In so doing, as Mother, she illuminates the path of our existence, supports us in our daily occupations and guides us towards the final encounter with Christ,' he said.
The pope invited everyone to live this season of hope and preparation with "great sobriety" and simple moments of family prayer.
"The situation we are experiencing, marked by the pandemic, generates concern, fear and despair in many; there is the risk of falling into pessimism ... How should we react to all this? Today's Psalm recommends to us: 'Our soul awaits the Lord: He is our help and our shield. It is in Him that our hearts rejoice,'" he said.
"Advent is an incessant call to hope: it reminds us that God is present in history to lead it to its ultimate end, to lead it to its fullness, which is the Lord, the Lord Jesus Christ," Pope Francis said.
"May Mary Most Holy, the woman of the expectation, accompany our steps at the beginning of this new liturgical year, and help us to fulfil the task of Jesus' disciples, indicated by the Apostle Peter. And what is that task? To render an account for the hope that is in us."