It appears that T. S. Eliot had become frustrated due to the limitations of Unitarianism and chose to make his opinion clear by expressing his perceived downsides of Unitarianism (including those on a cultural, intellectual and spiritual level) in his poems. According to him, Unitarianism was inadequate/incompatible with the rule of life and explanation of human existence that he truly identified with. This was reflected in "Preludes", specifically
"His soul stretched tight across the skies".
The shift from Unitarianism to Anglicanism represents Eliot's rediscovery of Christianity and is a core part of the understanding that there is order in the soul when an individual lives a life of virtue, when their thoughts and deeds fulfil the significant demands of their higher and nobler nature (i.e. God) and contribute to the perfection of their own nature. Eliot was essentially seeking God and failed to find Him in a distant Unitarian one.
In terms of the relevance of Eliot's conversion to your essay, religion plays an important role in gaining a better understanding of his poetry. This theme seems prominent in
"Journey of the Magi", which constitutes a reimagination of the story of the Three Wise Kings from the Bible. Given that the poem came during the period when Eliot was rediscovering his Christianity, its tone is somewhat optimistic. While the persona's journey (which is a reflection of Eliot's own journey) is rewarding, it reveals a degree of doubt. The questioning evident in "
were we led all that way for" and
"Birth or Death?" indicates to the persona that despite having reached a sense of growth, immediate gratification and/or wholeness are not gained.
I hope this helps!