'''Old Calendarists''' are groups of Orthodox Christians who are in various levels of "resistance" against the mainstream Orthodox churches, identified mainly by their insistence on the use of the Julian Calendar. They are to be distinguished from the mainstream churches which simply follow the [[Julian Calendar]] yet remain in [[full communion]] with most or all of world Orthodoxy. The term"Old Calendarist", however, is misleading, as can be seen below, since all Russian churches of any note, including the [[Church of Russia|Moscow Patriarchate]], use the older calendar, and so the calendar was never an issue. However, the Old Calendarists have, in general, always considered themselves spiritually united to the [http://orthodoxwiki.org/Old_Calendarists#The_Catacomb_Church_of_Russia Russian Catacomb Church], as defined below. In correspondence with and concerning each other they are usually styled as '''True Orthodox''' or '''Genuine Orthodox''', and both terms are used in this article. Some may also style themselves '''Traditionalist Orthodox'''.

Most Old Calendarists today say that their concerns about [[ecumenism]] , modernism (and in the case of the Russian Churches, "Sergianism") are more significant than calendar issues, which they believe have their origin in the former. The mainstream churches have varying positions on different Old Calendarist churches, ranging from an active desire to restore unity to a denial of the Old Calendarists' Orthodoxy.