Cædmon's Hymn is an unpretentious, uncomplicated 7th C hymn (spiritual poem) written in the alliterative style of its native Northumbrian AngloSaxon. It reveals the cosmology and spiritual world view of the Anglo-Saxon peasant, a voice of the illiterate that is usually unsung in the annals of ancient history.
This work is almost always referred to as a Christian hymn. It's not. This misperception results from an overly credulous reading of the work's very earliest citation - Bede's Historicia Ecclesiastica Gentis Anglorum, which obscures the hymn with a Latin substitute. We have an anonymous scribe to thank for defacing a copy of the Historia with the first AngloSaxon version of this hymn.
Definitively pre-Christian
Cædmon's Hymn is definitively pre-Christian because Frea is the deity named as 'almighty.' This name is often translated as "the Lord" but in the contemporaneous pre-Christian Germanic peasant society Cædmon grew up in, this is an explicit allusion to a pagan God-ruler derived from the Norse and Germanic tangle of Frey, Freya, Freyr, and Friga and is the basis for how Friday is named. Frea (Frey) is assigned to fertility and fruitfulness, not a small concern for anybody living a precarious existence in the fields and forests of the era.

Though depicted sometimes in public art as tonsured and habited, Cædmon was not a cloistered brother of Streaneshalch (Whitby) - that's a privilege only the rich could afford; he would have worked in his own clothing and unless he stayed in the village, he slept in the barn, a hut, or a bunkhouse. It is noted elsewhere that he was a gifted and popular singer, with appearances noted at what today might be called beer blasts. Other anecdotes regarding how this hymn came to be are fictitious and hardly bear repeating, save for their incidental acknowledgement that there is no first hand source for this hymn nor do we know that we have it in its entirety.
Speaking AngloSaxon
I want to make a note about various Youtube and podcast recitations of AngloSaxon in general and Cædmon's Hymn in particular: Speech flows smoothly around linguistic obstacles, and should never sound like you're thinking about it, working at it, or clearing a bone in your throat. If your recitation sounds overworked and labored, then your rendition still needs more rehearsal. Consonants impart meaning, but velarizations, palatals, and glottals emerged as a means to facilitate the flow of phonological sounds, not impede them. Listen to how smooth French, German, and Dutch sound today when spoken by a native speaker. So it was in the 7th C., and no honest purpose is served by making their speech sound primitive. Cædmon and his adherents were smooth talkers, and their language was musical.
Cædmon's abbey practiced Celtic Christianity.
Until it didn't.
References and further reading
Cædmon's Hymn (Scott Kleinman, Cal. State Univ., Northridge, 2007)
Cædmon’s Hymn A multimedia study, edition and archive (Daniel Paul O'Donnell, Univ. of Lethbridge, 2005)
Reading Cædmon's "Hymn" with someone else's glosses (Keven Kiernan, Univ. of Kentucky, online)
Monasticism in Anglo-Saxon England: an analysis of selected hagiography from Northumbria written in the years after the Council of Whitby (Carrie Couvillon, LSU, 2005 Masters Thesis)
The Cult of Kingship in Anglo-Saxon England: The Transition from Paganism to Christianity (William A. Chaney, University of California Press, 1970) Out of print. Google Books preview.
Paganism to Christianity in Anglo-Saxon England (William A. Chaney, The Harvard Thelogical Review, July 1960) via JSTOR (online version free)
The Medieval Latin Hymn (Ruth Ellis Messenger, 1953; ebook 2017 - hashlinked to #The Old Hymnal)
From Fate to God (Bob Trushaw, 2013 on Anglo-Saxon Twilight)
The Names of God (Lavinia Cohn-Sherbok, Reprinted from Judaism: A Very Short Introduction, OneWorld, 2005)
The Elder Gods: The Otherworld of Early England (Stephen Pollington, Anglo-Saxon Books, Little Downham (Ely), UK, 2011)
Recitation in West Saxon (mystanzachannel):
