It was the spirit of Christ that appeared after his death. His spirit had so much power that He could appear or rather be visible to humans.
There is a liteature in other religions about how this can happen. Same as our previous discussion of channeling.
The jews were not as sophisticated about the mechanics of the supernatural as other religions, except for Islam, which to me to be even less knowledgeable than Christianity, like in believing that heaven is where you are surrounded by many beautiful virgins. If you think about that, they are not very blissful to the dead man if they remain virgins.
Anyway, the doctrine of resurrection in my opinion is a product of the lack of sophistication of the jews. They had no concept of the soul. And especially no concept of the soul going to heaven. In Judiaism (except for the Kabala)when you are dead you no longer exist.
The Greeks introduced the concept of the soul and its possibility of going to heaven. Christ introduced this concept to Judiaism but retained the concept of ressurrection of the body. The two concepts: that of the soul and that of resurrection actually are contradictory. The soul retains the important characteristics of the body. Thus there is no need for the actaul physical body to come to life. That contradicts the laws of biology and thermodynamics.
So I guess bottomline, I think that concept is mainly rejected on the basis of science. Religion has always tried to be in agreement with known science at the time of its revelation. So I think we really need new religion that is now consistent with science as we now know it.
Perhaps a genius, a Messiah, will come along and teach us a religion that makes sense scientifically, including the science of the supernatural. God knows it all. He.she just has to pick some human to transmit all that information through.