Clerics appeared in Men & Magic, combining the advantages of fighting-men and magic-users, a hybrid between the two with the ability to cast healing spells (Gygax and Arneson 1974:8). Gygax described the cleric as a character capable of using both heavy weapons and powerful magic (1987:29).
The term cleric comes from a Biblical reference, in Deuteronomy xviii 2, to the Levites, members of an Israelite tribe whose men were assistants to the Temple priests: “Therefore shall they have no inheritance among their brethren: the Lord is their inheritance.” The Greek word for “inheritance” is “kleros” which became “klerikos” to reference matters relating to the Christian ministry. In ecclesiastical Latin it transformed into the word “clericus” which was borrowed into late Old English as cleric (Ayto 1990:117).
The cleric’s healing abilities hail from the divine connection bestowed upon bishops and the miracles they were able to perform. The cleric’s power to repel the undead had its roots in Dracula, which both coined the popular term “undead” and established a vampire hunter’s ability to turn away vampires by the presentation of a crucifix (a holy symbol).
The Dungeons & Dragons cleric was a religious warrior who disdained pointed weapons, wore armor, and invoked miracles. The cleric’s origins are drawn from the Bishop Odo of Bayeux, who was depicted in the Bayeux Tapestry as wielding a club-like mace at the Battle of Hastings. Popular perceptions of the time believed that the bishop wielded the mace to avoid shedding blood, as mandated by clergy taking a vow of non-violence. Odo’s brother Duke William was depicted carrying a similar item, suggesting that the mace may have instead been a symbol of authority (Aeon 2001).
Maces in any form are likely to shed blood. Nevertheless, the notion of a bishop wielding a weapon has precedent. Warrior bishops existed as recorded in church history as early as the 4th century of the Christian era. The Archbishop of Turpin wielded both spear and sword in The Song of Roland. Bishop of Adhemar of Le Puy also fought during the First Crusade.
This description is a rough draft from my upcoming book, The Evolution of Fantasy Role-Playing Games.