Here’s an outrageous question: Do we have good reasons to think that the apostles believed that Jesus was resurrected from the dead? I think the right answer is, we just don’t know what they believed, and we just don’t know what happened with Jesus.
Here’s why: the oldest existing copies of the Gospels that we now have are from about 200 CE to 370 CE. Our earliest fragments of John and Luke are from around 200 CE, our earliest copies of Mark are from 320 and 370 CE. It’s hard to get simple answers to these questions, and the older the manuscript is, the more likely it is just some fragments. It appears that the oldest copy of Matthew that we have is from the 300s as well.
There appears to be a consensus among experts (I am not one of them) that Mark was written about 70, Matthew and Luke were written around 80-90, and John was written around 90 CE. But even though scholars date the original authorship of those documents to those dates, we do not have any of those originals. We have copies of copies of copies of hand-me-down copies of those documents (we don’t know how many copies were made in between). For a number of obvious reasons, we should not assume that Mark(70) was the same as Mark(320). Even the enthusiastic proponents of the scholastic tradition concede that there will be a lot of drift over the course of 250 years. Matters are complicated by the fact that zealous, committed, deeply devoted believers were the sole proprietors of all of the documents for centuries—the fox has been guarding the hen house and we have only the fox’s word about the safety of the chickens.
The problem is even worse because the originals were written 30-60 years after the alleged resurrection of Jesus by people hearing about the stories by word of mouth after they had been repeated countless times. The originals were not written by the apostles Matthew, Mark, Luke and John.
So really most of what we have to inform us about what happened to Jesus and what the apostles believed are some fragments of some documents from 200-300 years after the fact. If we were to compile all of the passages from these oldest surviving documents that refer to the resurrection of Jesus in one place, they would maybe fill up a single page.
Back to the question: What is our evidence concerning what the apostles believed about the resurrection of Jesus? Answer: a tiny handful of crumbling fragments of documents from 2 to 3 centuries after the resurrection is alleged to have happened. Do you think that’s enough evidence for us to conclude anything with confidence about what they believed happened with Jesus? I don’t think that gives us enough evidence to conclude anything about the real events surrounding Jesus and his death.
Why focus on whether or not the Apostles believed that the resurrection happened? If they didn’t, wouldn’t it be utterly absurd that you do? If they didn’t, then what possible reasons could we have for believing that something happened that the eyewitnesses didn’t acknowledge? If the apostles didn’t believe it, then it wouldn’t be reasonable for anyone to believe it. If we have insufficient evidence to determine whether or not the Apostles believed it, then we should conclude that we just don’t know what happened with Jesus.
The additional problem here is that any reasonable person would acknowledge that generally speaking, extraordinary claims require extraordinary evidence. Claiming that Jesus was resurrected from the dead is extraordinary. We do not have a single plausible, corroborated case of such a thing ever happening in human history.
So in order to reasonably believe that Jesus was resurrected from the dead, a substantial burden of proof would need to be met. How much evidence do we have concerning the event? A single tiny handful of crumbling fragments of documents written 2 to 3 centuries after the fact. Believing that Jesus was resurrected or believing that some of his followers believed it on the basis of that evidence is unreasonable.
Again, I am not an expert on this material and the scholarship is a vast, tedious labyrinth. I may have some of the dates wrong, and I may have missed some important facts. But from what I can tell, in general outline the picture I am drawing is a roughly accurate map of what we now know about these historical sources. It won't really do to quibble about whether Mark was written in 70 or 65, or whether or not I Corinthians was written in 55 CE when the general problem I am describing involves tiny fragments of centuries old documents that were endlessly copied and retold stories from zealous converts.
It appears that the one thing we know for sure is that there is very little we know about the actual events or beliefs of the people who were involved in the death of Jesus.