"R.C." is commonly an acronym used for Roman Catholicism, so synonyms for this phrase would involve other terms and phrases that mean the same thing. Some possible synonyms include "Catholicism," "the Catholic Church," "the Holy Roman Church," and "the Apostolic Church." Each of these phrases refers to the religious faith and practices of the Roman Catholic Church, which is one of the world's largest and oldest religious institutions. As a result, these synonyms can be used interchangeably with "R.C." to describe the Catholic religion and its teachings, practices, and beliefs.