A frequently mentioned and important text in the Dune series is the Orange Catholic Bible. Having read the appendices for the first Dune, I understand why this book exists. However, I do not understand the naming of it. I would think that its name would reflect the combination of religions rather than use one only (Catholicism). And, on a lesser note, why orange?
Per the Dune Encylopedia, the name originally began as a joke (deriding the pomposity of those who felt that their new book was the literal be-all and end-all of religious thought, given that it contained elements from every religion) and just stuck.
The word appears to be a corruption of the original title "Koranjiyana" which if said quickly sounds like the word "Orange".
In this context, this doesn't refer (directly) to the Catholic faith but rather its common meaning, "encompassing everything".
In early fragments of his memoirs, Bertoli [a contemporary in-universe commentator] refers to it as the Koranjiyana Zenchristian Scriptures, or as the Zenchristian Navakoran, but after the fourth year it seems to be settled in his mind, at least, that Orange Catholic Bible was to be the name. We must suppose that a day or a week was given over to settling what may have become a matter of embarrassing dispute. The terms "orange" and "catholic" do, however, seem to have established themselves as reflecting the more innovative and rational as opposed to the more conservative and traditional schools of thought, sometimes being used quite lightly, if we may judge by odd remarks of Bertoli—"what a delightfully pompous catholic statement," "Catholic to a See," "utterly Orange is the only word to describe that nonsense," "for an Orange, that little acolyte of Bruin's is quite a peach," "all of those oranges are bananas"—remarks not at all clear to us now, but some were clearly meant as witticisms. The origin of the term Orange as applied to a religious sectarian is now obscure but its religious significance is so overwhelming nowadays that few remember it as the ancient name of a fruit now called portyguls.
In his biography of his father, Dreamer of Dune: The Biography of Frank Herbert, Brian Herbert, who has continued his father's work on Dune from his father's notes, writes (p. 187):
An Orange Catholic Bible is in the book, suggesting a future merging of Protestantism and Catholicism, ...
Brian Herbert was 18 years old when Dune was published, so he was at an age at which he probably lived at home while his father worked on the book. It is quite likely that his father talked to him about the book he was writing.
Brian Herbert is now also in the possession of several thousand pages of notes and background material on Dune, which probably makes him the best authority on the series.
There may not be a truly perfect or universal answer to this question, but may simply be a way to bring in the obscure into the Dune universe. It could also be a way of bringing opposites together to see a perceived reality.
The word Catholic means universal. The term is not limited to the Catholic Church as most know this term. The Eastern Orthodox Church is officially known as the Orthodox Catholic Church. Other denominations also claim the title catholic in their Official title. These churches do not see eye to eye on many points of doctrine, yet still claim the title catholic.
In Valorum's answer he quotes the following:
The origin of the term Orange as applied to a religious sectarian is now obscure.
Perhaps it is not all that obscure. Irish Catholic and Orangemen have been enemies since 1690.
The Twelfth (also called the Glorious Twelfth or Orangemen's Day) is an Ulster Protestant celebration held on 12 July. It originated during the late 18th century in Ulster. It celebrates the Glorious Revolution (1688) and victory of Protestant King William of Orange over Catholic king James II at the Battle of the Boyne (1690), which began the Protestant Ascendancy in Ireland. On and around the Twelfth, large parades are held by the Orange Order and Ulster loyalist marching bands, streets are bedecked with British flags and bunting, and large towering bonfires are lit. Today the Twelfth is mainly celebrated in Northern Ireland (where it is a public holiday), but smaller celebrations are held in other parts of the world where Orange lodges have been set up. The Twelfth involves thousands of participants and spectators, although not all Protestants celebrate it.
Orangemen commemorated several events dating from the 17th century onwards, celebrating the continued dominance of Protestantism in Ireland after the Irish Rebellion of 1641 and triumph in the Williamite War in Ireland (1689–91). Early celebrations were 23 October, the anniversary of the 1641 rebellion (an attempted coup d'état by Catholic gentry who tried to seize control of the English administration in Ireland), and 4 November, the birthday of William of Orange, Protestant victor of the Williamite war in the 1690s. Both of these anniversaries faded in popularity by the end of the 18th century. - The Twelfth (Wikipedia)
To demonstrate the unity of opposites, the Flag of Ireland has the green (color of the Catholic Irish and St. Patrick in particular) to the left and orange to the right (after the Orangemen of 1690). These colors are separated by a band of white (the color of truce).
Orange as a color is also symbolic in other region of the world:
In Ukraine in November–December 2004, it became the colour of the Orange Revolution, a popular movement which carried activist and reformer Viktor Yushchenko into the presidency. In parts of the world, especially Northern Ireland, the color is associated with the Orange Order, a Protestant fraternal organisation and relatedly, Orangemen, marches and other social and political activities, with the color orange being associated with Protestantism similar to the Netherlands. - Orange (color) [Wikipedia]
The word bible simply mean book or books.
In conclusion the Orange Catholic Bible seems to envision an obscure meaning of a Book of Universal Unity through Opposites?
The Orange Catholic Bible (abbreviated to O. C. Bible or OCB) is a fictional book from the Dune universe created by Frank Herbert. Its title suggests a merging of Protestantism and Catholicism, along with many other religious traditions. Created in the wake of the crusade against thinking machines Herbert calls the Butlerian Jihad, the Orange Catholic Bible is the primary orthodox religious text in the Dune universe and is described thus in the glossary of the 1965 novel Dune:
ORANGE CATHOLIC BIBLE: the "Accumulated Book," the religious text produced by the Commission of Ecumenical Translators. It contains elements of most ancient religions, including the Maometh Saari, Mahayana Christianity, Zensunni Catholicism and Buddislamic traditions. Its supreme commandment is considered to be: "Thou shalt not disfigure the soul." - Orange Catholic Bible (Wikipedia)