For most of history most worshippers could not read, and the substance of religious teaching reached them through song. The medium shaped the message in specific and traceable ways.
Melody makes text stick
Words attached to a tune are recalled far more reliably than spoken prose, and a melody supplies cues for what comes next when memory falters.
Rhyme and metre add further constraint, since a forgotten word must fit both sense and sound, which narrows the possibilities considerably.
This is why doctrinal summaries, narratives and moral instruction were so often versified. The form was chosen for transmission rather than for beauty alone.
Repetition delivered teaching gradually
A hymn sung weekly for years deposits its content slowly, without anyone sitting down to study it.
Congregations frequently know sung material far better than anything read to them, and can produce it decades later without having consulted a text.
The effect is cumulative and largely unnoticed, which is why disputes over changing familiar wording tend to be unexpectedly heated.
Communities defended their songs
Because sung material carried the teaching most people actually held, controlling it mattered to religious authorities in several traditions.
Reform movements often produced new bodies of song deliberately, on the understanding that changing what was sung would change what was believed more surely than changing what was preached.
Conversely, suppressed communities preserved their teaching in song precisely because it required no books and could be carried without evidence.
The form limited what could be conveyed
Verse suits narrative, praise, petition and memorable formulations, and handles qualification, argument and technical distinction poorly.
Teaching transmitted this way therefore tends towards the vivid and the concrete, with fine points left to those who studied texts directly.
Some traditions accepted this division openly, treating sung material as the general instruction and written commentary as the specialist layer.
Vernacular song reached people that liturgy did not
Where formal worship used a language most worshippers did not speak, songs in local languages carried the content that could actually be understood.
These often circulated alongside official liturgy rather than replacing it, and communities frequently held both without regarding them as competing.
Collections gathered much later preserve some of this material, though the versions recorded are usually a settled form of something that varied from village to village.