Earlier in the day, the Pope visited Sr Faustina's Chapel in the city of Kraków in southern Poland, where he met with the sanctuary's nuns.
While on the site, he heard the confession of five youths who attended the event.
Later, he travelled by Popemobile to the shrine of St JPII, situated some four kilometres south of the historic centre of Kraków.
In his homily of the Mass, attended by hundreds of faithful, Pope Francis said: “For us who are disciples, it is important to put our humanity in contact with the flesh of the Lord, to bring to him, with complete trust and utter sincerity, our whole being.
"As Jesus told Saint Faustina, he is happy when we tell him everything: he is not bored with our lives, which he already knows; he waits for us to tell him even about the events of our day.”
Following the mass, the Pope gifted Polish Cardinal Stanisław Dziwisz a mother-of-pearl cross which will remain as an exhibit at the shrine. The sanctuary was consecrated in 2013.
Priests attend Pope Francis celebrating the Holy Mass for Polish priests, men and women religious, consecrated persons and seminarians, in the Saint John Paul II Shrine of Krakow, Poland, 30 July 2016. Photo: EPA/DANIEL DAL ZENNARO
A relic of the late Polish pontiff John Paul II was present during the mass. St JPII, who was canonised in 2014, was the longest serving Pope in the 20th century and third longest in history.
Later in the day, Pope Francis was set to dine with 12 pilgrims who attended the World Youth Day: two from Poland and the rest from the other continents.
The Holy Father Francis' five-day-long visit to Krakow is in connection with the 31st World Youth Day planned in the city from 26-31 July. (rg)