The people who really believe this probably want to believe it for reasons that we can only speculate about. However, the best response I can think of to those who make this assertion is to ask them to "prove it."
The idea of Jesus being a Buddhist monk was started by a Russian Jew named Nicolas Notovitch who referred to himself as an adventurer, spy, and journalist. He published a book in 1894 entitled, "The Unknown Life of Jesus Christ, by the Discoverer of the Manuscript," which is based on the idea that Jesus left Galilee during his hidden years and traveled to India where he studied with Buddhists and Hindu's before returning to begin His public ministry in Israel.
The evidence he cites for this assertion is a Tibetan manuscript entitled "Life of Saint Issa, Best of the Sons of Men," which he found during a trip to India. (Issa is the Arabic name of Jesus in Islam.) Notovitch discovered the document while recuperating from a broken leg in the Hemis monastery in Ladakh, India.
However, this tale was declared a hoax almost as soon as it was published. Serious researchers such as J. Archibald Douglas, who was a professor of English and History and the Government college in Agra, interviewed the lead lama at the Hemis monastery who claimed a man named Notovitch had never been there, nor was there any such document about St. Issa in existence.
Other false accounts of Jesus' activity in India claim that there are documents detailing a man matching the description of Jesus who entered a Buddhist monastery while gravely injured and on the point of death. The monks nursed him back to health and he remained at the monastery for many years before he changed his name and went back into the world.
Another school of thought says Christians copied the history of Jesus from the history of Siddartha Gautama, the original Buddha. They like to say that just as Jesus was born of a woman who was visited by a divine being, Gautama's mother had a dream in which a white elephant entered into her and emerged nine months later as the "enlightened one." This is hardly similar. Nor is there any similarity in how Jesus was raised poor and Gautama was raised wealthy by a mother who was royalty. About the only thing they have in common is that they both left home (don't we all?) and began preaching.
The most critical difference between the history of Gautama and that of Jesus is that Gautama's legacy wasn't recorded until centuries after his death. Therefore, there is no eyewitness account of his life or teachings such as we have of Jesus.
I've also heard people speculate about how the teachings of Christianity parallel the teachings of the Buddha such as the 10 precepts being similar to the 10 commandments. However, these are not even remotely similar which makes sense because the basic philosophy of Buddhism is completely opposed to that of Christianity.
For instance, as I point out in this blog, Buddhists do not believe in the existence of the soul. They believe people who think they have a soul are rooted in ignorance and in a desire to please one’s “self” and that we become truly enlightened only after we come to the realization that there is no such thing as a soul. Christians not only believe in the existence of the soul, but that the soul can achieve eternal life through Jesus Christ. Buddhists believe in a reincarnation of sorts, but not of the soul. This reincarnation involves some element of one’s former identity.
Christians believe suffering brings us closer to God and unites us with our Suffering Lord. Buddhists believe suffering is something to be escaped from.
Christianity focuses on holiness, worship of God and restoring the relationship between God and man through Jesus Christ. Buddhists are not concerned with the existence of God but instead seek after “non-self” (anatman).
Christians believe that truth, and its Author, can be known rationally; Buddhism denies existential reality and believes nothing, not even the self can be proven to exist.
Christian prayer seeks to enter into a dialogue with God; Buddhist meditation strives to “wake” one from their delusions and to enter into altered states of consciousness.
I could go on and on, but I think you get the idea that claims of Jesus being a Buddhist are complete fiction.
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