Those who believe that Jesus was an actual historical figure claim that the originality of his story and his teachings prove that he had to have been a real person. They fail to recognize that the teachings of Jesus are hardly original. For example, Buddha told his followers to "love your enemies" hundreds of years before Christ was born. I'll deal with the sayings of Jesus in a future post, for now, let's turn to his biography.
Aspects of Jesus' story can be found in numerous myths. Cuneiform tablets from 1500 BCE tell the story of the Sumerian goddess Inanna (also known as Ishtar) who was crucified and resurrected. In the fifth century BCE, Herodotus reports that the Thracian god Zalmoxis buried himself alive, was resurrected three years later, and founded a religion which preached immortality in the afterlife.
According to literary sources such as the Bhagavat Purana, Krishna, like Christ, was a divine incarnation, the product of immaculate conception, and was of royal descent. Devatas (similar to angels) sang songs of praise at his birth and he was visited by shepherds. The reigning monarch, fearing that the infant would overthrow him, put all infants to death, but Krishna escaped to another country. He also raised the dead, cured lepers, and washed the feet of Brahmins. While there is no literary source, there is also a tradition that Krishna was crucified based on monuments and sculptures.
Similarities between Jesus and other mythic heroes have been pointed out ever since the early days of Christianity. Celsus criticized Christianity for being too similar to the other Graeco-Roman mystery cults of the time. Scholars have found similarities between Jesus and numerous other gods including Orpheus, Kore, Mithras, Attis, Adonis, and Osiris.
We know very little about the mystery cults since they kept much of their rites secret. What little we do know is often derived from modern interpretation of reliefs, sculptures and paintings. However, early Christians such as Irenaeus and Justin Martyr found the similarities so overwhelming they accused demons of founding the mystery cults before Christ was born in order to spread doubt about Christ.
But those who hand down the myths which the poets have made, adduce no proof to the youths who learn them; and we proceed to demonstrate that they have been uttered by the influence of the wicked demons, to deceive and lead astray the human race. For having heard it proclaimed through the prophets that the Christ was to come, and that the ungodly among men were to be punished by fire, they put forward many to be called sons of Jupiter, under the impression that they would be able to produce in men the idea that the things which were said with regard to Christ were mere marvellous tales, like the things which were said by the poets. [...]
The prophet Moses, then, was, as we have already said, older than all writers; and by him, as we have also said before, it was thus predicted: "There shall not fail a prince from Judah, nor a lawgiver from between his feet, until He come for whom it is reserved; and He shall be the desire of the Gentiles, binding His foal to the vine, washing His robe in the blood of the grape." Genesis 49:10 The devils, accordingly, when they heard these prophetic words, said that Bacchus was the son of Jupiter, and gave out that he was the discoverer of the vine, and they number wine among his mysteries; and they taught that, having been torn in pieces, he ascended into heaven. [...]
And when they heard it said by the other prophet Isaiah, that He should be born of a virgin, and by His own means ascend into heaven, they pretended that Perseus was spoken of. And when they knew what was said, as has been cited above, in the prophecies written aforetime, "Strong as a giant to run his course," they said that Hercules was strong, and had journeyed over the whole earth. And when, again, they learned that it had been foretold that He should heal every sickness, and raise the dead, they produced Æsculapius.--Justin Martyr, First Apology
While much of the practices of the mystery cults remain hidden, we do have some information. The Greek god Dionysos, for example, was born of a virgin, died and came back to life, and his followers enjoyed a sacred meal of raw meat and wine symbolizing their god's flesh and blood. In the tragic play Oedipus written by Seneca, we learn that Dionysos turned water into wine for his wedding with Ariadne. He wasn't thought of as a fictional character either. According to Plutarch, the people of Delphi believed the remains of Dionysos were buried near their oracle.
This doesn't necessarily mean that Christians plagiarized from the other religions, but it certainly shows that the story of Jesus follows many universal mythical themes with very little originality to be found.