I had a look for the 12 Days over Christmas, but not here, and came across this explanation (or that it was used by covert English Catholics) in a lot of places, but with none of them giving a source. There is something odd about it. Firstly, in both England and Ireland, the Catholics were surrounded by Protestants, and a lot of the coded information was shared with them. There was no need to hide the Pentateuch in five gold rings, or the gospels as calling birds. There is very little specifically Catholic in there. Where is any reference to devotion to the Virgin? What about the numbers of sacraments? These are the things one would expect to be hidden. In England, (and I hadn't seen the Irish suggestion, where I know the situation was very different), after the Tudors, when some Catholics were executed for treason (the Pope had issued a sort of fatwa on Elizabeth), many Catholics were able to continue as such without being too secretive. The Norfolk family, and others, were able to do so, having private chapels, and hold high office. Some even did so during Elizabeth's reign. In the same way, (without the offices), Nonconformists at the other end of the spectrum were able to continue without encrypting their teachings. In most cases in the song (the gospels being a notable exception), the mnemonic is not easily related to the coded details by anything except the number. You might as well use playing cards. I would very much like to see the earliest references to this being used for the purpose given. One source suggested a Commonwealth origin. Given that Christmas itself was banned for a while there, hiding sacred information in a Christmas song at that time seems a little foolish. Especially when, at that time, anyone could get away with singing a number song enumerating the Testaments, the virtues (people named themselves from them), the gospels, the Pentateuch, the days of creation, the gifts of the spirit, the beatitudes, the fruits of the Spirit, the Ten Commandments (part of every parish churches decoration allowed to remain), and the eleven faithful disciples in the open.