TL;DR: Maryland ceased to be much of a haven for Catholics by the 1650s. The Church of England was the official, established state religion in the colony from 1692 until the Revolutionary War. There continued to be some degree of religious tolerance there, though Catholics did face legal repression, too. Nevertheless, nearly two centuries passed between the end of Maryland's status as a haven for Catholics and the 1856 presidential election when the Know-Nothings won the state's electoral votes.

LONG ANSWER:

1. Colonial History of Catholicism in Maryland

Maryland was founded in 1632 as a "proprietary colony" of Cecil Calvert, Lord Baltimore. Lord Baltimore did found it as a haven for English Catholics (being a Catholic himself), although this wasn't explicitly put in writing. Instead, the Maryland colonial charter granted by the King of England, included something in it often referred to as the "Durham Clause". (Most of the English "proprietary colonies" would include variants on this clause.) This was in reference to the Bishopric of Durham back in Britain, which essentially had given that bishopric broad authority over the governance of Durham. Likewise, Lord Baltimore was granted all the same privileges and immunities over colonial Maryland that the Bishop of Durham had over Durham.

In other words, Lord Baltimore was allowed to establish courts and a legislature, hire governors, etc., however he saw fit. The colony was sort of his own personal kingdom. Technically speaking, the King of England was still the ultimate authority, but as long as the King was receiving his stipulated cut of the profits from the colony and the colony wasn't causing any problems, then Lord Baltimore could do what he wanted.

So, the King of England was the official head of state and church in Maryland, which meant that Maryland was implicitly Protestant since the King was Protestant. But since the Lord Baltimore had broad power, he could establish the colony as a haven for English Catholics if he wanted, and the King would not interfere. And so that's what Maryland was at first. The earliest colonists were Catholic, although even by the end of the 1630s, there were many Protestants who lived there, too, side by side with the Catholics.

In 1639, the Maryland assembly passed their first explicit religious tolerance law (the "Act for Church Liberties"), followed ten years later by the more well-known and elaborate "Toleration Act of 1649". This latter act is often cited as the first law protecting religious liberty in the future United States. However, as historian Kenneth Lasson writes, critics often point out that it wasn't very tolerant, since one of the sections of the law was to make blasphemy against the Catholic church a crime, punishable by a fine or by the whip.

In any case, by the time of the 1649 law, the English Civil War had already begun. In reaction, Lord Baltimore had appointed a Protestant governor of the Maryland colony (William Stone), and this latter law was put in place largely to protect the Maryland Catholics against the impending threat of a Puritan takeover. But it didn't actually work. The law was repealed five years later, after the Third English Civil War. Shortly after, Puritans successfully deposes the Protestant, King-supporting William Stone, who was taken prisoner in 1655 after the Battle of Severn (the only battle of the English Civil War fought on American ground). Lord Baltimore then replaced him with another Protestant, Josias Fendall, but he, too, was arrested by the Puritans who had taken over Maryland. He was allowed to return to England as long as he promised not to interfere in the Maryland's government.

But after the Restoration, Fendall returned to power and returned to Maryland, whereupon Maryland was established as a de facto Anglican colony, with King Charles II as head of church and state. Maryland did still attempt to return to its practice of religious tolerance of all Christians, but after the death of Lord Baltimore in 1675, his successors were never able to exert much Catholic power over the colony.

In 1687, King James II ascended to the throne, and attempted to establish Catholic rule over England and its colonies. This led to the Glorious Revolution where he was deposed. The repercussion in Maryland was the Establishment of the Anglican Church in 1692, as not just the "in name only" official church of Maryland, but a church with actual power over religious matters. Although, as historian Maura Jane Farrelly writes, due to a dispute over the wording of the Maryland law that had nothing to do with religion, the Establishment of the church didn't actually become written law in the colony until 1702. Nevertheless, Farrelly goes on to describe the situation for the duration of Maryland's colonial history:

"Between 1692 and 1776, the [Maryland] General Assembly took up numerous pieces of legislation as part of a collective effort to restrict the civil, military, educational, economic, religious, and even parental rights and behavior of Maryland’s 'papists.' Even when these bills were not actually turned into laws—and they were, frequently, turned into laws—the discussions and debates that the bills engendered made it decidedly inconvenient, at times even onerous to be Catholic in Maryland.

"In spite of the antagonistic cultural and legislative environment, Catholicism survived in Maryland—and the Catholic population itself grew more than fivefold throughout the Penal Period—because the people who made up the Catholic community refused not to be Catholic."

Long story short, by the time the Know-Nothings won the presidential electoral votes in Maryland in 1856, the colony/state's status as a safe haven for Catholics had been over for the better part of two centuries. The Church of England had been the established state church there since 1692, or 164 years until that election when Millard Fillmore won the state. To put it in perspective, conveniently, moving forward to the present day, it's been 164 years since that 1856 election. So it's a bit like asking, "How did the Catholic Joe Biden win the 2020 presidential election in Maryland if that state had gone for the Know-Nothings back in 1856?" Everything can change, politically speaking, over the course of 164 years.

Thus, while the historical background is always important, there were more immediate reasons why the Know-Nothings won Maryland in 1856 that were far removed from the colony's first couple decades as a Catholic haven in the 1630s and 40s.

2. The Success of the Know-Nothing Party in the Southern Border States in 1856

By the time the Know-Nothings were able to win Maryland's electoral votes in the 1856 Presidential election, the party had largely been co-opted by former Whig Party politicians looking for a new home. To summarize some of the writings of Whig Party historian Michael F. Holt, the Know-Nothings became that vessel more out of convenience than anything. The Know-Nothings had already begun attracting anti-Catholic, anti-immigration, "Bible Belt" (as it would have been understood at the time) former Whigs as the Whigs collapsed in the early 1850s, especially with the 1854 mid-term election. As Holt writes, this anti-immigrant/anti-Catholic voting bloc was very much one of the reasons that the Whigs collapsed. The Whigs had been the home of the anti-Catholic vote, but had never been so vehement about it, while with the Know-Nothings it was the central philosophy of the party. Another wing of the Whigs were more motivated by the anti-slavery, "free soil" philosophy, and established the Republican Party at the same time.

Holt does point out that it would be a mistake to think that the Know-Nothing Party was simply a reassembling of the Whig Party. They certainly did believe in that anti-Catholic platform, and weren't just trying to rebrand themselves. However, there certainly were also ex-Whigs who very much joined out of expediency, to get elected/retain their Congressional seat, and once elected, did not push the anti-Catholic, anti-immigrant agenda very hard. They focused on other issues, often old Whig platform positions.

Further, the anti-Catholic, anti-immigrant faction of the party was concentrated where Catholics and immigrants lived. That is, mostly in the urban North and Midwest, in places like New York City, Boston, and Cincinnati, which had large populations of Irish and German Catholics.

Elsewhere, the Know-Nothing Party voters were motivated more by their desire to keep Democrats out of office. As Holt writes:

"...many conservatives voted for Millard Fillmore in 1856, and the desire to preserve the union was probably a major source of his strength in the South and the border states [such as Maryland]. Know Nothingism in many southern states was essentially a continuation of the Whig party under another name, a vehicle through which Whigs could contend with their long-time foes, the Democrats, for control of state governments and congressional seats."

With the Whigs no longer viable and not running their own presidential candidate in 1856 (the last year they ever held a national convention), the Know-Nothings did well in places like Maryland and other border slave states. These were states where the anti-slavery Republicans wouldn't fly, but there was a strong pro-union, pro-nationalist sentiment there as well, so the Democrats weren't universally liked, either. So, Millard Fillmore could do pretty well. He won Maryland over Democrat James Buchanan 55-45%, while the Republican John C. Fremont only got a few hundred votes.

cont'd...