The Mythic Theory
The disciples themselves made the claim
The resurrection can be traced to the original disciples. You can say they lied, you can say they saw things that weren't really there, but stating that the resurrection was a legend that developed over time isn't an option. The disciples made the claim immediately after Jesus' death in Jerusalem, the city where Jesus was killed.
Paul and James revisited
Paul himself claimed to become a Christian not on the basis of hearing some myth, but by what he believed to be an appearance of Jesus.
James, Jesus' brother also came to believe in the resurrection based on an appearance of Jesus, apart from the testimonies of the disciples. While the mythic theory is popular on the internet, it just doesn't explain the evidence.
The early church fathers revisited
In the early second century, Justin Martyr wrote On The Resurrection, which is written as a defense of the bodily resurrection. One hundred years later, the early church father Origen of Alexandria wrote another defense against the pagan philosopher Celsum, who wrote a scathing attack against Christianity and the resurrection. These responses presented arguments for the view of a real, bodily resurrection. Why would they make these arguments, if a real, embodied resurrection wasn't proclaimed?
What about dying and rising gods in other religions?
To that, I simply say...what about them? For the reasons we already looked at, the comparisons are just a distraction to what is actually being claimed for Christianity - that Jesus was a real man, who died and rose again. Stating that there are dying and rising gods in other religions say nothing about the historical evidence we're examining.
"One cannot imagine how such a series of legends could arise in an historical age, obtain universal respect, and supplant the historical recollection of the true character [Jesus]....if eyewitnesses were still at hand who could be questioned respecting the truth of the recorded marvels. Hence, legendary fiction, as it likes not the clear present time but prefers the mysterious gloom of gray antiquity, is wont to seek a remoteness of age, along with that of space, and to remove its boldest and most rare and wonderful creations into a very remote and unknown land".
- Julius Müller, 19th-century Polish professor, and theologian. Studied law and theology at Göttingen University.