Pope Francis celebrates Mass at Madison Square Garden in New York City, Sept. 25, 2015. (Alan Holdren/CNA)

NEW YORK CITY (CNA/EWTN News) - Pope Francis on Friday had a brief, but urgent, reminder for the Catholics of New York City. They must proclaim the joy of God and remember to care for all those who go unnoticed in their metropolis, because they have seen the "great light" of Jesus Christ.

"Knowing that Jesus still walks our streets, that he is part of the lives of his people, that he is involved with us in one vast history of salvation, fills us with hope," the Pope said during his homily at Mass at Madison Square Garden Sept. 25.

"God is living in our cities. The Church is living in our cities, and she wants to be like yeast in the dough. She wants to relate to everyone, to stand at everyone's side, as she proclaims the marvels of the Wonderful Counselor, the Mighty God, the Eternal Father, the Prince of Peace," he added.

His homily, delivered in Spanish, drew from the prophet Isaiah's declaration: "The people who walked in darkness have seen a great light."

"We ourselves are witnesses of that light," the Pope continued. "God's faithful people can see, discern and contemplate his living presence in the midst of life, in the midst of the city."

The Pope reflected on the multicultural nature of large, modern cities, with such "hidden riches" as diverse cultures, traditions, languages, and cuisine.

"But big cities also conceal the faces of all those people who don't appear to belong, or are second-class citizens," he said. There are people who go unnoticed, like foreigners, children without education, those without health insurance, the homeless, and the forgotten elderly.

"These people stand at the edges of our great avenues, in our streets, in deafening anonymity. They become part of an urban landscape which is more and more taken for granted, in our eyes, and especially in our hearts," he said.

Pope Francis said that when people in the Gospels asked Christ "what must we do?" the first thing he did was "to propose, to encourage, to motivate."

"He keeps telling his disciples to go, to go out. He urges them to go out and meet others where they really are, not where we think they should be. Go out, again and again, go out without fear, without hesitation. Go out and proclaim this joy which is for all the people," the Roman Pontiff encouraged the congregation of tens of thousands.

"Go out to others and share the good news that God, our Father, walks at our side. He frees us from anonymity, from a life of emptiness and selfishness, and brings us to the school of encounter. He removes us from the fray of competition and self-absorption, and he opens before us the path of peace."

This peace, he said, is "born of accepting others," and fills our hearts "whenever we look upon those in need as our brothers and sisters."

The Pope also reflected on God as the Everlasting Father. 

"No one or anything can separate us from his Love. Go out and proclaim, go out and show that God is in your midst as a merciful Father who himself goes out, morning and evening, to see if his son has returned home and, as soon as he sees him coming, runs out to embrace him."

Pope Francis said the knowledge of Christ's presence fills Christians with "a hope which liberates us from the forces pushing us to isolation and lack of concern for the lives of others, for the life of our city."

This is "a hope which frees us from empty 'connections,' from abstract analyses, or sensationalist routines. A hope which is unafraid of involvement, which acts as a leaven wherever we happen to live and work. A hope which makes us see, even in the midst of smog, the presence of God as he continues to walk the streets of our city."

The Pope will be in the United States until Sunday. On Saturday, he will travel to Philadelphia, where he will visit a correctional facility and take part in the World Meeting of Families.