Williams's #1 motivating factor was to save the Dutch Republic from being forced to fight isolated against the France of Louis XIV. James II of England sought an alliance with France that would have left the Dutch all on their own and Louis had begun to turn the screw on the Netherlands by initiating trade sanctions etc.

As a Catholic, James started out facing a serious handicap. He made a few mistakes, but his alliance with France was an enormous blunder. It wasn't so much that France was Catholic (though it certainly aggravated his image problem) as the fact that it was France. The match struck many in England as simply bizarre.

The rumours circulating regarding the legitimacy of James' son added to his troubles - and William was married to James' protestant daughter Mary. William cannily decided that a bold stroke against England might shatter the cohesion of James' kingdom and pull things out of the fire for the Dutch and that was the fundamental motivating factor for his invasion.

While the statements in the letter regarding the attitude of the papacy to the subsequent war are correct, James' attempt to use Ireland as a springboard to re-take England was a desperation move that embroiled him in a straight-up sectarian fight.

William himself had no special animus against Catholics that I know of, though Protestants of whatever sect didn't tend to look well on what they regarded as a religion riddled with superstition. What's more, England and then Britain would be allied with reactionary-Catholic Austria for much of the coming century.

The idea of being invaded from Ireland would have appalled many English people, however, though the religious element was only part of that.

After William's death, a succession of measures collectively known as the Penal Laws were enforced in Ireland. These mainly targeted Catholics but Presbyterians were also subject to more moderate religious discrimination. OTOH, the stigma against Catholics in England itself, which ran very deep since Guy Fawkes' time, gradually faded as the 18th Century progressed and there was actually never a time when Catholics were entirely absent from senior positions in the British body politic.

Presbyterians did feature prominently among the United Irishmen and the abortive rebellion of 1798 though much of the leadership was actually Episcopalian, including Lord Edward Fitzgerald and Wolfe Tone. In fact many Presbyterians continued to support Home Rule for Ireland up to the 1880s, such as John Martin who defeated the a Unionist, Catholic 'Mr. Plunckett' (running with the backing of the clergy, apparently) in the Meath by-election of 1871.

Over the coming decades, this situation would be transformed. There would continue to be a significant Protestant cadre of Irish nationalists, including Charles Stewart Parnell, but very few if any of them were Presbyterians. I can't pin down the reason for the surge in sectarian sentiment -- which certainly wasn't restricted to Protestants Edit: as the glee in some quarters at Parnell's downfall showed -- but it can be said that if Pitt's plan was to use the Orange Order to drive a wedge between Catholics and Presbyterians it took a long time to have any effect. The disestablishment of the (Episcopalian) Church of Ireland in 1869, which eliminated the tithe payable to that church, eliminated one source of Presbyterian grievance.

It's doubtful William of Orange would think very highly of the organisation that bears his title -- in his time, Holland was a model of religious toleration relative to practically anywhere else. Neither was sectarian sentiment a major feature in England by the end of the 19th Century. Anti-Irish bigotry -- exemplified by some famously vicious Punch cartoons of the late Victorian period -- was a huge motivator for at least a significant strand of British opinion, though.

The Catholic church in Ireland also became increasingly strident. The high water mark of Catholic supremacism wouldn't come until the 1930s/1940s, however. It reached its zenith not so much from universal support as from a craven failure to resist in almost all quarters -- Irish politicians of that era had long forgotten the often anti-clerical tone of Irish nationalism of the 1910s.

I take issue with the idea that William III was not anti-Catholic. The Penal Codes in part existed before William ever came to England, but the most extremely anti-Catholic portions came into being while he was still King. He saw his war against France as not only a liberating war for the Dutch Republic, but also a conflict with strong religious dimensions. He did put aside a lot of his sentiments on religion for the sake of politics, which is why he allied himself with some Catholic powers, including Spain and the Papal States. However, once his throne was secured, he also made alliances with Protestant hardliners in Ireland. In fact, they were part of the reason that he eliminated some of the more strenuous disabilities from Irish Presbyterians and other dissenters. It was under William that the Acts were passed to prohibit Catholics from many areas of civil service, instructing the youth of the country, possessing any firearms, learning to make firearms, intermarrying with Protestants, or to be educated at Trinity. He also renewed a discontinued code that prevented Catholics from becoming members of Parliament or addressing the Parliament. For a fuller listing of Catholic disabilities under William III, see the University of Minnesota's law library website.

Also, he did create a great deal of anti-Catholic propaganda against France and later, Ireland. It is true that the Dutch Republic was tolerant for its age, but anti-Catholicism was still a strong facet of seventeenth-century Dutch politics and culture. During the period from 1647 to 1702, Jonathan Israel has noted that restrictions on Catholic worship and the creation of 'hidden chapels' created a segregation that was fundamental to the fabric of Dutch life. "Toleration" in the early modern context was quite a different animal than its twenty-first century equivalent.

That being said, focusing more on the issue of the National letter, it is wrong on several counts. First, the idea that the Presbyterians "dropped their bigotry towards Irish Catholicism" is just plain wrong. Many Orangemen and women have been Presbyterians over the years. The letter's author made the mistake of assuming that because some Presbyterians supported the United Irishmen that sectarianism was at a low point. The fact is that the Orange Order was formed in 1795. While some Protestants were supporting the United Irishmen, others were preparing to fight them. Sectarian episodes were common, especially due to the proliferation of agrarian secret societies like the Heart of Steel Boys, Peep O'Day Boys, Defenders, etc; 1798 also had some nasty sectarian moments in which Catholics and Protestants killed each other in extremely interesting ways.

I have never ever seen the quote by Brigadier General Knox that is mentioned here and is absent in a lot of the literature about the Orange Order. Even in the conspiracy-theory tin-foil-hat version of the Order's history lacks that particular quote.

As far as the British ruling classes using the Orange Order to divide the classes, this is also untrue. The Orange Order was suppressed by the British government in 1825 and a parliamentary inquiry regarding the Order was completed in 1935. The several volume reports make for interesting reading, but the bottom line is that the Order was deemed an embarrassment by many British politicians. This embarrassment was set aside when the Orange Order could be of use to London, for example during the Home Rule Crises of 1886, 1893, and 1912. Northern Irish politicians have been a little more directly linked to the Order, but politicians elsewhere in the UK don't really like to claim them.

Sources

Barnard, T. C. A New Anatomy of Ireland: The Irish Protestants, 1649-1770. New Haven: Yale University Press, 2003.

Blackstock, Allan. Loyalism in Ireland, 1789-1829. Woodbridge: Boydell Press, 2007.

Cronin, Mike. A History of Ireland. Houndmills, Basingstoke, Hampshire: Palgrave, 2001.

Doyle, Mark. Fighting Like the Devil for the Sake of God: Protestants, Catholics and the Origins of Violence in Victorian Belfast. Manchester: Manchester University Press, 2009.

Farrell, Sean. Rituals and Riots: Sectarian Violence and Political Culture in Ulster, 1784-1886. Lexington: University Press of Kentucky, 2000.

Israel, Jonathan I. The Dutch Republic: Its Rise, Greatness and Fall, 1477-1806. Oxford: Clarendon Press, 1995.

Mijers, Esther, and David Onnekink. Redefining William III The Impact of the King-Stadholder in International Context. Aldershot: Ashgate, 2007.

Troost, Wouter. William III the Stadholder-king: a political biography. Aldershot, Hants, England: Ashgate, 2005.

Great Britain. Report from the Select Committee Appointed to Inquire into the Nature, Character, Extent and Tendency of Orange Lodges, Associations or Societies in Ireland: With the Minutes of Evidence and Appendix. [London]: [House of Commons], 1835.

Great Britain. Second Report from the Select Committee Appointed to Inquire into the Nature, Character, Extent and Tendency of Orange Lodges, Associations or Societies in Ireland: With the Minutes of Evidence and Appendix. [London]: [House of Commons], 1835.

Great Britain. Third Report from the Select Committee Appointed to Inquire into the Nature, Character, Extent and Tendency of Orange Lodges, Associations or Societies in Ireland; With the Minutes of Evidence, Appendix and Index. [London]: House of Commons, 1835.