The Pope made the address to what's thought to be more than a million people, most of them young people, at a special prayer vigil dedicated to mercy in Brzegi, a village outside Krakow, on Saturday night.
He entered the large field holding with young people from across the world before they all walked through a door which says "Jesus, I trust you" in several different languages.
The pontiff said: "Dear young people, we didn't come into this world to 'vegetate', to take it easy, to make our lives a comfortable sofa to fall asleep on.
"No, we came for another reason: To leave a mark."
"Following Jesus demands a good dose of courage, a readiness to trade in the sofa for a pair of walking shoes and to set out on new and uncharted paths.
"The times we live in do not call for young couch potatoes, but for young people with shoes, or better, boots laced."
On Sunday Pope Francis will lead a final Mass in Krakow and announce where the next World Youth Day in three years time will take place.
Speaking to Premier Joseph Phillip, one of the young Christians at World Youth Day, said: "We can't just explain it in words. I'm pretty sure if you ask anyone here and myself, I would say I don't want to leave.
"It feels so much like family. Just being on tubes, everyone is singing, everyone is praising the Lord.
"For me what it opened up was the Church, the strength of the Church, what we are as a family.
"Sometimes when you're back home there aren't a lot of young people... when the whole nations come together you see it's not just you."
Listen to Premier's Aaron James speaking to Joseph Phillip here: