While the conspiracy claims made by Brown and Gardner might not spark visceral outrage, their attacks on the Catholic Church are outrageous nevertheless. One cannot read Gardner, or even Brown, without concluding that the framework of these Grail societies is anti-Catholic in outlook.
Think of it this way: In America and Great Britain, the issues of the reformation taints the view that Protestants have toward Catholic structures, while Catholics themselves harbor resentment for having mean nuns teach them catechism in Junior High. In our population, the Roman Catholic Church is highly visible. With that visibility and influence comes both high regard and deep resentment.
Among those who harbor resentment, it goes without saying that the Roman Catholic Church is voted most likely to be guilty of villainy and subterfuge. We absorb a novel like the Da Vinci Code and take for granted its premise that “the Vatican” had a monopoly on power as early as the Nicene Council (Brown, p. 254). We completely (and conveniently) forget the reality on the ground for the Christians of the Eastern Mediterranean at the time these alleged Grail documents would have been produced and hidden. These Christians of the Eastern Mediterranean had governance over the reliquaries of Palestine including anything to do with the Temple of Solomon. They also happened to be Greek-speaking, not Latin, and they esteemed the Patriarch of Constantinople more highly than they esteemed the Latin-speaking Pope of Rome.
The facts of history spoil the romantic image of a villainous, all-power "Vatican". The romantic image has greater emotional appeal, however, because readers in the United States and United Kingdom are more aware of the Pope and his influence, then of the Patriarch of Constantinople and the rich history of the Eastern Orthodox Church to the Nicene Creed and earlier.
Recent history is cause to have Rome even more prominent in western minds. After all, Constantinople fell to the Muslim Turks in the mid-15th
Century, and the city was renamed Instanbul. This shortened the influence of the Patriarch of Constantinople and of Eastern Orthodox Christianity. Another blow befell the Eastern Orthodox centuries later, when many of its adherent cultures were swept behind the iron curtain of Communism, a movement that very nearly took hold of Greece itself.
Persecuted and marginalized much like the church in the New Testament, the Eastern Orthodox do not register on the minds of western readers of popular fiction: thus, the Eastern Orthodox do not present convenient targets for conspiracy theorists and Grail enthusiasts. The churches of the East do not attract and capture the western imaginations to which these books are marketed.
Despite its centuries of hardship in the midst of Islamic or atheistic oppression, the Eastern Orthodox Communion remains a vibrant faith, claiming adherents throughout the Middle East and Russia. If you ask a family of Palestinian Christians, as to their Christian communion or denomination, they will most likely tell you they are Orthodox.
Brown and Gardner are trapped between implausibilities. Throughout history there have been extremely powerful groups that would have profited greatly from the Church’s embarrassment or even demise. In the East, where many of the documents originated and, according to Brown, were discovered in the ruins of Jerusalem, the documents would have been published first by Muslim Jihadists and then by Communists. This obviously does not fit in the mythology espoused by Brown.
So it is safer for the esoteric Grail Quest to ignore the role of the Eastern Orthodox in historic and Palestinian Christianity. It is safer to focus on romantic villains rather than on historical plausibility. It is safer to have Crusaders remove the documents from Jerusalem to secret locations in France and Scotland, because these locations are safely "western" and viscerally associated with the Roman Catholic Church. To accomplish the assumption that such a discovery of secret documents is even possible, one must ignore Islam's religious agenda during 3 centuries of rule over Jerusalem. One must also pretty much ignore the existence of Eastern Orthodox Christianity itself.
Only in this way can the Roman Catholic Church rise to its villainous prowess in the esoteric Grail quest and in Dan Brown's novel. Makes it fun to read, though, right?