The notion that Jesus wasn't crucified originally came from the gnostics, who believed that all matter is corrupt and that perfection could only be realized in the spirit. Jesus dying on the cross, in their minds (and in the minds of Muslims), would have been a failure on the part of God...and God doesn't make mistakes or have failures. So a number of traditions arose. One is that God did an end run around those who sought the death of Jesus. He turned the tide on them by making Judas look like Jesus, and spiriting Jesus away. Thus, the Romans crucified Judas while Jesus, unseen, laughed at them and their folly.
Of course this was heresy to the apostles and disciples, who taught that Jesus died for the sins of mankind (see Isa. 53) and was resurrected three days after his crucifixion, and that Judas committed suicide. The so-called Infancy Gospel, where Jesus, as a boy, cursed everyone who got in his way, and learned restraint as he grew older, also was popular reading and despite the fact it was a complete forgery, it found its way into the Qur'an along with the gnostic teachings. In fact, that's one of the "gotcha" points that prove the Qur'an is fiction. In fact, the author of the Infancy Gospel may have simply been writing it as a fictional account, even though it was very popular in Christian circles.
The bottom line is that the story of Jesus not being crucified is nothing new. Many saw the fall of Adam and Eve as being a failure on the part of God. Then, if Jesus' enemies succeeded in killing him, then that, too, would have been a failure. The Greek Orthodox position is that Adam and Eve were supposed to fall. For only by so doing could Jesus not only give them immortality, but "Eternal Life," as esoteric term meaning not immortality, but immortality with God, and co-inheritors with Christ. Thus, everything that Jesus became would be gained by everyone gaining Eternal Life. Thus, the fall and the resurrection were inseparately linked as a springboard to something that Adam and Eve never could have achieved on their own.
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