An arrogant, prideful, egoistic, religious person who believes his/her religion is the only true religion. They are deeply religious in their dogma/religion to such an extent they believe that is who they are. They are often times very legalistic and think they are better than others, especially other people who aren't members of their particular religion. People who are religionists are usually the biggest hypocrites.
Religious people are often times religionists due to the effect their religion has had on them over the years.
Jenny is so prideful about her religion, she is such a religionist.