1. What is the year 1975 on the religious calendar of the Roman Catholic Church, and how often are such celebrations held?
1975 of our Common Era—the year marked in the religious calendar of the Roman Catholic Church for celebration as a Holy Year, which really began on Christmas Eve, December 24, of the year 1974.
... 2. Current statistics show what as to the size of Christendom?
2 The Roman Catholic Church entered the Holy Year for 1975 with an estimated Roman Catholic population of 551,949,000, making it the largest religious organization on earth today.