Salmau yr Eglwys yn yr Anialwch (1812)

Iolo was a tireless promoter of Unitarianism in south Wales, and composed a vast corpus of hymns for Unitarian congregations. Of some three thousand hymns, only a fraction were published in Salmau yr Eglwys yn yr Anialwch (1812) and subsequent editions.

In general, the hymns are uninspiring and are heavily moralistic and theological in tone. Iolo also composed the tunes to many of his hymns.

The introduction to his collection is not typical of a religious publication. He gives fulsome thanks for the support he had received from leading Unitarians such as Andrew Kippis, Theophilus Lindsey and John Prior Estlin, but also cryptically refers to his (by then) sworn enemies, namely Owain Myfyr and William Owen Pughe. The introduction also resonates with themes which properly belonged to Bardism: Welsh metrics and the superior merit of his regional dialect. Furthermore, the volume was published not under the name Edward Williams, but 'Iolo Morganwg, B.B.D.'

Iolo also translated a primer for the Unitarian cause, Edward Williams, Holiadur, neu Addysgiadau Cyffredin, Hawl ac Atteb, yn Athrawiaethau Dyledswyddau Crefydd (Merthyr Tudfil, 1814).