As a former specialist in Celtic studies at the Sorbonne, he researched pre-Christian and medieval culture and spirituality.
His 'creative' use of scholarship and his tendency to make great leaps in reasoning cause scholars following more normative and conservative methods to balk and his interest in subjects that his critics consider questionable, including various branches of the occult, have gained him at least as many detractors as admirers.
The Breton scholar Christian-Joseph Guyonvarc'h dismissed Markale as follows; "Mr Jean Bertrand, a.k.a.
He never says where he teaches; but [...] he cannot properly accentuate Greek, knows nothing of Latin [...] he doesn't know how many cases there are in Irish declension (sometimes he says two, at other times three) [...] Jean Markale very complacently quotes his own works in his later publications and, every time an Irish text is mentioned, he refers the reader to his 'Celtic Epics' as though that book included actual translations or constituted the most basic and essential reference on the matter.
"[1] However, for others, his loose scholarly presentation is balanced by "the insightful points Markale does make about various texts, clever interpretations of certain scenes and thought-provoking parallels to other traditions".