Actually it as an edict from the King of England, prior to the formation of the United States of America, that prohibited the ownership of guns by Catholics in the Maryland colony in the mid-1700s. Further, if you go into the history of some of the original colonies, many were set up as havens by/for specific religions when they were first founded. Over a period of time, trade between the colonies increased tolerance of other religions and by the time the colonies became states religious freedom was recognized (but not always practiced).
However, all that said, the Brady Campaign (aka Handgun Control), and its lawyers recently cited the royal decree banning Catholic ownership of handguns when they filed a brief in the Washington DC gun ban case, which was recently overturned by the courts.
American Rifleman was not wrong it what it wrote, but they wrote more than was quoted in the posts in this thread, and the AR article more accurately described the situation. Clearly the Brady Campaign was digging deep to justify support for the Washington DC gun ban.