This Sunday, the Church commemorates her birthday. Christianity began with the Resurrection of our Lord and Savior Jesus Christ. What followed was a period of fifty days where the apostles and other disciples basically hid out in fear for their lives.
Despite being with Jesus for the roughly three years of his ministry, they had their doubts about just who he really was. They had to have been the densest holy men ever to walk the face of the earth.
Jesus, being the Incarnate Word, the Son of God, knew all about their weakness and promised to one day send them an Advocate to help them. That Advocate was, of course, the Holy Spirit. On Pentecost Sunday, the Holy Spirit descended upon them and gave them the wisdom to know what to say and do, and the strength and courage to go forth around the world and spread the Gospel.
This event marks the birth of the Church. That institution has continued under the leadership of holy (some not so much) men in an unbroken line from St. Peter through Benedict XVI, all guided by the Holy Spirit.