Did the group Coldplay really originally have the words "Roman Catholic priests are singing" in their song "Viva la Vida", and later replace it with "Roman Cavalry choirs are singing"?
This is a classic example of a misheard lyric (sometimes known as a "mongdegreen"). If you read the lyrics on Coldplay's own website and compare it with the original song you can hear the original text is "Roman cavalry choirs are singing". This is a fairly obscure reference (made a bit clearer by the reference to missionaries two lines later) and it's easy to understand why a lot of people mishear this as "Roman Catholic choirs are singing".
For a similar discussion see this question.