He described two temptations Christians must avoid in order to be a "new" people and receive the gifts of the Holy Spirit, before paying tribute to those affected by the terror attack in London on Saturday night.
The pontiff said: "May the Holy Spirit grant peace to the whole world, may He heal the wounds of war and of terrorism, which even this [Saturday] night, in London, struck innocent civilians: let us pray for the victims and their families."
Speaking to members of faithful under a near-cloudless sky, the pope highlighted what he saw as the importance of diversity and unity in the Church.
The coming of the Holy Spirit upon the disciples at Pentecost is told in the book of Acts.
He said: "[F]irst, [the Holy Spirit] he rests on each [of the disciples] and then brings all of them together in fellowship, giving each a gift for the good of the new community He has created.
"The same Spirit creates diversity and unity, and in this way forms a new, diverse and unified people: the universal Church."
"The first temptation seeks diversity without unity. The opposite temptation is that of seeking unity without diversity.
"The prayer we make to the Holy Spirit is for the grace to receive His unity, a glance that, leaving personal preferences aside, embraces and loves His Church, our Church.
"It is to accept responsibility for unity among all, to wipe out the gossip that sows the darnel of discord and the poison of envy, since to be men and women of the Church means being men and women of communion."