Section Four of Buddhism's Relation to Christianity contains the fascinating and important story of the adoption of Buddha, by the name of "Josaphat," into the Christian pantheon of saints, For several centuries in the Middle Ages up to the 19th, Christendom heard the marvelous tales of Josaphat and Barlaam, until the Age of Enlightenment brought with it Buddhist studies that revealed the ruse.
As Lockwood (61) remarks, "The Christian legend was, in actuality, a transmutation of the life of the Buddha," a transmutation evidently occurring sometime between the third and seventh centuries AD/CE.
This fact of a transmutation of a non-Christian godman into a Christian saint is not limited to Buddha, as it occurred with several other figures from pre-Christian religion and mythology, including St. Ann, St. Lucy, St. Denis and St. Brigid, representing the deities Anna/Ana, Lucia/Lucina, Dionysus and Brighid/Brigid/Brigit/Bride, respectively.
If enterprising monks (or their priestly, pontifical or political leaders) were busy transmuting a Buddhist legend into a Christian tale as early as the third century, there is little reason to doubt that their predecessors were busily transmuting other Buddhist stories and doctrines into Christian tales and dogma—including the gospel story itself, which only emerges clearly and in detail into the historical record during the latter half of the second century—a mere decades before the Josaphat fabrication evidently was begun....
Source: Buddhism's Relation to Christianity