From YOUCAT, fact 118:
118: What happened on Pentecost?
Fifty days after his Resurrection, the Lord sent the Holy Spirit down from heaven upon his disciples. The age of the CHURCH began.
On Pentecost the Holy Spirit transformed fearful apostles into courageous witnesses to Christ. In a very short time, thousands had themselves baptized: it was the birthday of the Church. The miracle of the languages on Pentecost shows that the Church is there for all peoples from the very beginning: She is universal (= the Latin term for the Greek kat’ holon, catholic) and missionary. She speaks to all men, overcomes ethnic and linguistic barriers, and can be understood by all. To this day the Holy Spirit is the “soul” of the Church, the essential principle of her life.