Simplify the following text for a Grade 9 native English-speaking person.
Original: December 25 on that calendar currently corresponds to January 7 on the more widely used Gregorian calendar . However , other Orthodox Christians , such as the churches of Greece , Romania , Antioch , Alexandria , Albania , Finland and the Orthodox Church in America , among others , began using the Revised Julian Calendar in the early 20th century , which corresponds exactly to the Gregorian Calendar .
Simplified: Most Christian countries of the world use a calendar called the Gregorian Calendar , but some churches use a calendar called the Julian Calendar .