"It is a beautiful thing" to build the Kingdom of God day by day, the Pope said. "Not to destroy; to build!"
Francis said the disciple of Jesus who undertakes this mission should be prepared to face difficulties and hostilities.
"Hostility is always at the beginning of the persecution of Christians," he said, "because Jesus knows that the mission is an obstacle against the work of evil."
It is for this reason that the disciples in the Gospel are sent out without money bag, sack, or sandals, in order that they might "rely only on the power of the Cross."
"This means abandoning every reason for personal pride, careerism, or hunger for power," the pontiff said, so that they may be "humble instruments of salvation operated by Jesus' sacrifice."
To be a Christian in the world "is a wonderful mission," Francis said, one which serves everyone without exclusion.
The Pope reflected on the need for Christians to carry out this mission joyfully.
"There is a great need for Christians to witness the Gospel with joy in their everyday life," he said.
Referring to the line in the Gospel that the disciples "returned rejoicing" from their mission, he said: "This expression makes me think of how often the Church rejoices, is made happy when her children receive the Good News thanks to the dedication of many men and women who announce the Gospel daily," be they priests, sisters, consecrated persons, or missionaries.
Francis concluded by issuing a challenge to the young people in the square to listen to God's word, and follow it. "Do not be afraid! Be courageous, and carry this torch of apostolic zeal which has been given to us by these exemplary disciples."
Update, July 4, 2016 at 11:27 GMT+2: The death toll and number of wounded were adjusted in the article to reflect the most recent reports. The article originally stated that around 80 were dead and more than a hundred had been wounded.