If I can add something: what was the Pope's reaction to the rebellion?

There's evidence for both sides being supported by Catholics.

The pro-Britain side arises from many factors, including the political system of monarchy being "approved" in Catholicism as it proctors Catholicism and allows the Church to regulate it (Church and state separation and freedom of religion happens at Vatican II with Dignitatis Humanae).

Another factor was the Continental Congresses' letters to spur allies in the Revolutionary War. One letter was sent to Catholic Canada, promoting religious liberty and causes of that nature, saying Britain was against it. Problem was, Britain actually gave Catholics religious freedom and voting rights in the Quebec Act. The CC letters to England's Parliament, families, and colonies, however lambasted the Quebec Act and feared the encroaching Papism on American soil.

The latter three letters were published in English. The Canadian one: in French.

Well....the English letters got translated and published in Canada. They were offended and now less likely to get involved in the War. Also, the King just gave them religious freedom to be Catholic, so they were pro-Britain.

The other side (pro-American) arose out of a common Protestant heritage and religious freedom. Jay Dolan argues American Catholics were like sponges, saturating themselves with Protestant culture and ideas. One idea was religious toleration, a view arising out of a Lockeian worldview of religion, where toleration is due to one being unable to transmit knowledge of God truly to another.

The Catholics in Maryland, the first Catholic presence in the United States' area, has a tumultuous history with the local Protestants resulting in a civil war. This pissed off Cecilius Calvert, who wanted to make money off of tobacco sales in his Colony. Over 70% of the working population fled or died. Calvert was a capitalist in nature so e wanted money. So he outlawed discrimination.

Mention hating the Virgin Mary in public?Whipped and fined. Deny Jesus as Lord? You are put to death. What Calvert was trying to was make hatred private. Like a grandfather saying anti-Islamic things in private, the child learns not to say those things in public.

These measures allowed toleration for all Christians provided one pledged loyalty to Calvert. One man was convicted of the law. Being a Jew, Jacob Lumbrozo obviously denied Christ as Lord. But he pledged loyalty to Calvert, which acquitted him of all charges.

This religious toleration made Catholics in Maryland see their religious identity differently than their political ones. Although short lived, it did its job in ensuring that Catholics saw themselves as American and as independent from Europe.

American bishops also argued for independence and transparency. John Carroll, the first bishop of Baltimore, argued for an English Mass in a time of Latin liturgy....so the people could understand it. The American Catholics also practiced trusteeism, where they controlled secular Church affairs usually done by European bishops.

If you want to read a book on most of the things I said, try Maura Jane Farrelly's Papist Patriots: The Emergence of an American Catholic Identity. Good synthesis of information about the era. It's also important to note the anti-Catholic sentiments aren't due to a hatred of the Catholic religion; to many, Catholicism was able to be tyrannical and the Americans hated tyranny. That's a post for another time though. As a Catholic as well, I thought the Founders hated Catholics, but if that was so, the First Amendment would not have allowed free toleration.

To add something to this, the Papists Act was passed in 1778, allowing Catholics to purchase and inherit land among other things. Catholics in England and her colonies could have seen this as the beginning of the end of the Penal Laws. To gain the benefits of this law one had to swear an oath to the King, so you could see that this might persuade Catholics to throw their support to the British.

I think /u/MacduffFifesNo1Thane has the heart of the answer here: Loyalty often depended on where a Catholic lived. Marylanders had learned to fuse their religious and political/national identities. In areas where Catholics remained persecuted minorities, they tended to support the side that could best guarantee their toleration. Remember that the Patriot cause was strongest and most commonly identified with New England. While not as Puritanical as they had been in the 17th century, New Englanders still tended to be proudly, violently Protestant. Few sane Catholics would trust their lives and liberties to these religious fanatics who had no qualms about directing violence at political opponents.

As such, some Catholics did actually take up arms to fight for the Crown. Here's some documents on the Roman Catholic Volunteers, a loyalist unit that was formed in 1777.

What's ironic is that while in many parts of America, Catholics fought for the Crown, they were the Demographic that most won out when the Revolution ended.