Pentecost is often called the birthday of the Church because on that day the apostles were filled with the Holy Spirit. The Holy Spirit, however, did not merely come to visit but to stay! The Spirit's work was not only to get the apostles started as the Church but to remain with the followers of Jesus, giving life and guidance to the Church forever.
One example of the effect of the presence of the Holy Spirit in the Church can be found in the life of the first Christian community. In the period of time that followed the birthday of the Church on Pentecost, the early Christians were so united in their love for Jesus and for one another that everbody shared everything with one another.
Together, the Christian community followed the teaching of the apostles. They prayed daily, and they celebrated the "breaking of the bread," which was the early form of the Mass. The early Christians were a minority, but they impressed everyone by their way of life.
The power of the Holy Spirit pushed the apostles and the early Christiansto continue to invite more and more people to believe in Jesus and in the saving power of his death and resurrection. Groups of Christians were formed in more and more cities and towns.
At this point, however, only members of the Jewish religion were invited to become Christians. They were God's chosen people, and they believed in the one true God. They were the people to whom God had promised a Saviour, and Jesus was the promised Savior. At first, therefore, the apostles had the idea that it was not necessary, or even right, to preach about Jesus to anyone who was not Jewish. After all, the other people did not even know the true God. They still had their own false gods.