Every single time I find a direct parallel between medieval European thought and D&D, I ask Gary about it on one of our Q & A threads. Every time, he states that no research was done and it is sheer coincidence.
There are two original lives of St. Cuthbert, both of which are an easy read. One is by the Venerable Bede, another by an unknown author. Both were written in the early 8th century. While Cuthbert also appears in Bede's Ecclesiastical History, the original Life is definitely the thing to read. I found Bede's version in J F Webb's 1965 Lives of the Saints and the anonymous (and in my opinion more D&D-ish) in Clinton Albertson's 1967 Anglo Saxon Saints and Heroes.
There are plenty of great stories of saints doing Christian magic in the early medieval period that make excellent fodder for D&D. I'm attaching a list of sources for especially magic-oriented saints' lives:
Adomnan of Iona. The Life of Saint Columba Trans. Richard Sharpe. (London: Penguin Books, 1995.)
Anonymous. "The Life of Saint Cuthbert" Trans. Clinton Albertson in Anglo-Saxon Saints and Heroes. (Fordham: Fordham University Press, 1967. 31-86.)
Athanasius of Alexandria. "The Life of Saint Antony" Trans. David Brakke in Medieval Hagiography Ed. Thomas Head. (London: Routledge, 2001. 1-30.)
Bede. "The Life of Saint Cuthbert" Trans. J. F. Webb in Lives of the Saints. (Harmondsworth: Penguin, 1965. 69-130.)
Bede. A History of the English Church and People Trans. Leo Sherley-Price. (London: Penguin Books, 1968.)
Einhard. "The Translation and Miracles of the Blessed Martyrs Marcellinus and Peter" Trans. Paul Edward Dutton in Charlemagne's Courtier: The Complete Einhard. (Peterborough: Broadview Press, 1998. 69-130.)
Felix. "The Life of Saint Guthlac" Trans. Charles W. Jones in Saints Lives and Chronicles in Early England. (Ithaca: Cornell University Press, 1947. 125-160.)
Gregory of Tours. Glory of the Martyrs Trans. Raymond Van Dam. (Liverpool: Liverpool University Press, 1988.)
Gregory of Tours. Life of the Fathers Trans. Edward James. (Liverpool: Liverpool University Press, 1985.)
Gregory the Great. "The Life and Miracles of Saint Benedict" Trans. Odo John Zimmerman in Medieval Saints: A Reader Ed. Mary-Ann Stouck. (Peterborough: Broadview Press, 1999. 167-204.)
Head, Thomas, Ed. Medieval Hagiography (London: Routledge, 2001.)
Muirchu. "The Life of Saint Patrick" Trans. A. B. E. Hood in Saint Patrick: His Writings and Muirchu's Life. (London: Phillimore and Company, 1978. 61-98.)
Severus, Suplicius. "The Life of Saint Martin of Tours" Trans. Carolinne White in Early Christian Lives. (London: Penguin Books, 1998. 129-160.)
Stephanus, Eddius. "The Life of Saint Wilfrid" Trans. Clinton Albertson in Anglo-Saxon Saints and Heroes. (Fordham: Fordham University Press, 1967. 87-164.)
Strabo, Walahfrid. "Saint Gall" Trans. Maud Joynt. in Medieval Saints: A Reader Ed. Mary-Ann Stouck. (Peterborough: Broadview Press, 1999. 223-249.)
Willibald. "The Life of Saint Boniface" Trans. Clinton Albertson in Anglo-Saxon Saints and Heroes. (Fordham: Fordham University Press, 1967. 297-314.)