I can't put my hand on a specific reference to this, but there was a suggestion that singers practising for church or chapel didn't want to profane the sentiment of the proper words by stopping and starting and concentrating on getting the singing right. To avoid this, they sang secular lyrics to practise the musical side of things, using the religious lyrics at the appropriate place and time.

This is certainly true. My wife comes from a small Scottish village in the South West - where her father was minister.

They used a song called the "Old Seceder Cat". Elsewhere this is sung to Lyngham - a tune also used for WSWTFBN on a regular basis.

There's a link and details here:

here