“In the general spoliation of Buddhism to make up the new Christian religion, it was not to be expected that so peerless a character as Gautama-Buddha would be left unappropriated. It was but natural that after taking his legendary history to fill out the blanks left in the fictitious story of Jesus, after using what they could of Christna’s, they should take the man Sakya-muni and put him in their calendar under an alias. This they actually did, and the Hindu Saviour in due time appeared on the list of saints as Josaphat, to keep company with those martyrs of religion, SS. Aura and Placida, Longinus and Amphibolus.
In Palermo there is even a church dedicated to Divo Josaphat. Among the vain attempts of subsequent ecclesiastical writers to fix the genealogy of this mysterious saint, the most original was the making of him Joshua, the son of Nun. But these trifling difficulties being at last surmounted, we find the history of Gautama, copied word for word from Buddhist sacred books, into the Golden Legend. Names of individuals are changed, the place of action, India, remains the same, in the Christian as in the Buddhist Legends. It can be also found in the Speculum Historiale of Vincent of Beauvais, which was written in the thirteenth century. The first discovery is due to the historian de Couto, although Professor Muller credits the first recognition of the identity of the two stories to M. Laboulaye, in 1859.
Colonel Yule tells us that these stories of Barlaam and Josaphat, are recognized by Baronius, and are to be found at page 348, of The Roman Martyrology, set forth by command of Pope Gregory XIII., and revised by the authority of Pope Urban VIII., translated out of Latin into English by G. K. of the Society of Jesus. To repeat even a small portion of this ecclesiastical nonsense would be tedious and useless. Let him who doubts and who would learn the story, read it as given by Colonel Yule.
Some of the Christians and ecclesiastical speculations seemed to have embarrassed even Dominie Valentyn. “There be some, who hold this Budhum for a fugitive Syrian Jew”, he writes, “others who hold him for a disciple of the Apostle Thomas; but how in that case he could have been born 622 years before Christ, I leave them to explain. Diego de Couto stands by the belief that he was certainly Joshua, which is still more absurd.” “The religious romance called The History of Barlaam and Josaphat was, for several centuries, one of the most popular works in Christendom”, says Colonel Yule. “It was translated into all the chief European languages, including Scandinavian and Sclavonic tongues…” This story first appears among the works of St. John of Damascus, a theologian of the early part of the eighth century.”
Here then lies the secret of its origin, for this St. John, before he became a divine, held a high office at the court of the Khalif Abu Jafar Almansur, where he probably learned the story, and afterwards, adapted it to the new orthodox necessities of the Buddha, turned into a Christian saint.”
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 