Today is Pentecost Sunday, the day when Christians recall the descent of the Holy Spirit upon Christ's disciples after his ascension into heaven.
When the time for Pentecost was fulfilled, they were all in one place together.
And suddenly there came from the sky a noise like a strong driving wind, and it filled the entire house in which they were.
Then there appeared to them tongues as of fire, which parted and came to rest on each one of them.
And they were all filled with the Holy Spirit and began to speak in different tongues, as the Spirit enabled them to proclaim.
Now there were devout Jews from every nation under heaven staying in Jerusalem. At this sound, they gathered in a large crowd, but they were confused because each one heard them speaking in his own language.
They were astounded, and in amazement they asked, “Are not all these people who are speaking Galileans? Then how does each of us hear them in his native language? We are Parthians, Medes, and Elamites, inhabitants of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the districts of Libya near Cyrene, as well as travelers from Rome, both Jews and converts to Judaism, Cretans and Arabs, yet we hear them speaking in our own tongues of the mighty acts of God.”
The Gospel at Mass shows a sort of "little Pentecost" while Christ was still on earth. The last line of the passage is a basis for the Catholic belief in the Sacrament of Reconciliation.
From John Chapter 20:
On the evening of that first day of the week, when the doors were locked, where the disciples were, for fear of the Jews, Jesus came and stood in their midst and said to them, “Peace be with you.”
When he had said this, he showed them his hands and his side.
The disciples rejoiced when they saw the Lord.
Jesus said to them again, “Peace be with you. As the Father has sent me, so I send you.”
And when he had said this, he breathed on them and said to them, “Receive the Holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained.”
The image above is from here.