J.C. Robertson 
History of the Catholic Church 

The beginning of the Christian Church is reckoned from the great day on which the Holy Ghost came down, according as our Lord had promised to His Apostles. At that time, "Jews, devout men, out of every nation under heaven, " were gathered together at Jerusalem, to keep the Feast of Pentecost (or Feast of Weeks), which was one of the three holy seasons at which God required His people to appear before Him in the place which He had chosen.