How did Tchaikovsky's 1812 Overture become part of American Independence Day celebrations

On the other hand, the Catholic Council of Trent in 1572 ruled the other way, forbidding the use of secular melodies in settings of the mass, which was widely popular in the 16th century.

I think the religious hymns in the 1812 are Orthodox, though, not Catholic. (You can also hear “God Save the Tsar”, the national anthem of the Russian Empire, at one point)

“My Country, 'Tis of Thee” is sung to the same tune as the British National Anthem…?

Which was either rescinded or ignored, because lots of Catholic hymns have tunes which originated from secular songs.