For this response, I'm going to write only about what scholars of history can determine from documentary material. I'm not going to take a position one way or another on the theological questions involved, as this isn't the place for that.

As far as I know from my Christian upbringing, when it comes to authors of the New Testament, Matthew, Mark, Luke, John, James, Jude and Peter all supposedly knew Jesus intimately for years.

I'm afraid your Christian upbringing left a few gaps. The traditional Christian understanding is that Mark and Luke were not written by anyone who knew Jesus personally. The gospel of Luke even starts out with an explanation that he is collecting accounts of what happened from others, that the author himself is not an eyewitness.

What we have are a number of writings regarding the early Christian movement. Some of them claim to be written by particular people, while others are anonymous. Unfortunately we have no writings that scholars generally recognize to be from anyone who ever met Jesus.

The majority position of scholarship today is that Jesus, Peter, and Paul almost certainly existed. (Though be aware that there is a small number of scholars who have concluded that Jesus himself was a myth. I don't find their arguments persuasive, but you should at least know that they exist.)

There are many different gospels of Jesus, but here I'll focus on the big four, the ones that were most accepted by early Christians and are now included in Bibles today.

Scholars today almost all recognize that Mark was the first gospel to be written, a position known as Markan priority. We don't actually know the name of the author for sure -- all four canonical gospels are anonymous -- but it was attributed to Paul's assistant John Mark around a hundred years after the time of Jesus.

Most of the material in Mark is copied in Matthew and Luke, not just the events, but the particular phrasing, with many stretches of word-for-word agreement. For a number of reasons (that I'd be happy to get into) scholars are generally convinced that Matthew and Luke both had access to manuscripts of Mark to copy from, along with one or more other sources that are now lost to us.

The gospel of John is generally believed to be the last of these four gospels composed, around 60 to 80 years after Jesus' day. Within a generation or so it was attributed to the apostle John. Most scholars are unconvinced of this authorship -- the sophisticated Greek writing, for example, seems unlikely to be the work of an Aramaic-speaking fisherman.

The two letters attributed to Peter are also in fluent Greek, while Peter is described in the book of Acts as being an illiterate fisherman. 1st Peter was generally accepted by the early church, but 2nd Peter's authenticity has been disputed since at least the time of Origen, writing around 200 AD.

Peter himself is one of the more widely attested figures from the New Testament, being mentioned by many different early Christian writers.

The letter of Jude is a very brief text in polished Greek. It does give the name of an author, Judas the brother of James. (Jude and Judas are two different English renderings of the same Greek name.) Unfortunately we're not sure exactly who this is supposed to be -- the author of Luke mentions a Judas, brother of James, but there is some dispute as to whether this is the same person as Matthew and Mark's Judas, brother of Jesus or John's Judas (not Judas Iscariot).

Jude was not universally accepted as valid by the early church. Some, such as Tertullian and Clement of Alexandria, accepted it, while others, such as Eusebius and Origen, did not. One sticking point was that Jude quotes the book of Enoch as an authority, a popular book of the day that most Christians no longer consider canonical.

The letter of James is similarly difficult to attribute to an author. Three of the gospels state that two of Jesus' disciples were named James, and a number of different texts mention James the brother of Jesus. We simply don't know which one of these the letter of James is supposed to have been written by, and the different Christian traditions disagree on this point.

A number of early Christian writers mention at least one James, though there is often some confusion as to the identity of the different Jameses. Josephus, a non-Christian writer of the first century, mentions James the brother of Jesus being among those stoned to death by the Sanhedrin for breaking the law.

As you mentioned, Paul himself never met Jesus. Paul claims to have had an extraordinary experience of Jesus, which is also reported with varying details in the book of Acts. Whether you believe Jesus actually revealed himself to Paul is a good question, but not the sort of thing that historians can determine.

Paul is mentioned by many early Christian authors, like Peter. (And like Peter, I'm not aware of any non-Christian authors who mention Paul early on.)

Overall, I think we have abundant documentary evidence that Paul, Peter, and Jesus existed. We have many different writings that speak of them that are probably from within a hundred years or so of the events described, and we have manuscript fragments of some of those writings from not much later.

As for theories about how people could come to believe that Jesus was raised from the dead without an actual resurrection, this quickly gets into religious questions, but historians can (and sometimes do) speculate. Maybe Jesus actually came back from the dead. Maybe Peter, out of grief and trauma, had a vision of Jesus. Religious beliefs sometimes spread very quickly. As for whether those beliefs are founded on truly miraculous events, I'll leave that to you to decide.

/u/trampolinebears focused on your second set of questions, about the authorship of the gospels. I'm going to address your first set, about the origins of the belief in Jesus' resurrection itself.

Before saying anything else, though, if you're looking for a great book-length study of this issue, you absolutely can't do better than Dale Allison's The Resurrection of Jesus: Apologetics, Polemics, History, published last year. Allison, one of the most respected scholars of early Christianity, by both Christians and non-Christians alike, has done absolutely superb work on this issue, and synthesizes the data and arguments in a singular and engaging way.

Onto your set of questions.

First off, your intuitions about the unlikelihood of Jesus' followers "colluding" to knowingly fabricate this idea of his resurrection, despite not actually believing this themselves, are correct. Offhand, I can't think of a single scholar today, religious or secular, who considers it seriously. You also correctly surmise that many reject this idea in light of the early Jesus-followers' apparent willingness to die in their proclamation; but this isn't the only counter-argument, or even the main one.

It's hard to explain this succinctly, but a lot of it has to do with the Jesus having already created a distinct and firmly established movement. Even before Jesus' death, his followers seem to have placed their utmost trust in him, as an expression of their own individual hopes in his teachings and in the future reality that he promised. They seemed to have abandoned their livelihood to follow him; and statements and teachings ascribed both to Jesus himself and to his followers in fact explicitly discuss this idea of unconditional support (Mark 8:35; Matthew 26:33; John 13:37).

What I'm trying to emphasize here is that they had first convinced themselves that Jesus was a singular figure, perhaps divine (cf. Ehrman 2014; Grindheim 2011), and who was to inaugurate the end of history itself (Allison 1998). Conversely, when we think of actual swindlers who try to invent a religion or cult de novo, we tend to think of those who have no real preexisting emotional and ideological stake in the belief itself. Instead, they invent the thing for largely utilitarian reasons, with a kind of sadistic desire to exploit the gullible or a fairly transparent profit motive. So the disciples' belief in Jesus' resurrection has a certain continuity with other preexisting strongly-held beliefs about him that also came about for reasons unrelated to trying to convince some external population of their reality or swindle them. Rather, it seems to be have emerged as an internal matter and belief of those within the Jesus movement itself, not influenced by external considerations.

But there are also a few caveats to all this; and discussing these opens up a few other important issues on the broader subject.

First, though, one of the reasons the work of Dale Allison (the scholar whose book I mentioned at the beginning) is so important is because he's one of the few scholars of early Christianity who's really devoted extended attention to examining the idea of the resurrection and its origins in a wider crosscultural and sociopsychological context. That is, he's tried to make sense of the idea and its emergence in light of other attested visionary experiences, and altered states of consciousness. (The terminology differs — "hallucination," religious experience, etc. — but the ideas are similar.)

And this connects precisely with the disciples' preexisting conviction in Jesus' teachings and mission, which I already discussed. The idea here, then, is that the notion of Jesus' persistence after death may have been influenced or facilitated by their conviction that Jesus could not have truly failed to fulfill his personal mission and the promises to bring about a new eschatological reality, without some decisive act on his behalf by God — especially if Jesus were a sort of preternatural, divine figure in his own right. So scholars who look at this aspect suggest that this unconditional conviction could have had a profound psychological and indeed perceptual effect for them in relation to Jesus' death, further influenced by their spiritual beliefs about the dead and their fate in general.

Dale Allison gets even more specific here; and in the course of examining this idea in its broader crosscultural context, he looks at these apparent resurrection experiences in conjunction with those who've had spiritual experiences or perceived postmortem "encounters" with loved ones while grieving for them in the wake of their deaths, e.g. in dreams or dreamlike states. This has been a fairly popular little sub-area of interest for those who've considered the resurrection experiences as the result of altered perception/an altered state of consciousness; and you can also find other specialty articles on this, too. For example, the abstract for Stephen Smith's recent article "The Post-Resurrection Appearances of Jesus as Bereavement Experiences: An Engagement with Gerald O’Collins" reads in part

In recent years, debate has resumed regarding whether or not the post-resurrection appearances may have been hallucinations or delusions on the part of the disciples. A sub-category of this debate is the question of bereavement hallucinations. Was the disciples’ bereaved state in the wake of the crucifixion responsible for their ‘seeing’ Jesus?

This comment is already getting a bit long, but I had mentioned some caveats to my earlier paragraphs. It's unfortunate that the New Testament gospels and other closely related documents are pretty much the only relevant surviving literary documents we have on this, since it's so hard to be able to cull "objective" historical data from these, in light of their ideological and apologetic aims, and their indulgence in what we can call fictionalization.

And this has direct bearing on pretty much all major things relevant to the origins of the disciples' resurrection belief and experiences. Here are a few of them, in no particular order:

1) Although it's one thing for a single individual to have a hallucinatory experience, it's often quite another when we're talking about shared visionary experiences and encounters. However, the latter is exactly what the gospels suggest in terms of Jesus appearing and interacting with the disciples as a group; though again, Dale Allison covers this exhaustively.

2) Did the historical Jesus himself proclaim that he'd die and be raised from the dead in three days (whether or not this ended up actually coming to fruition)? Certainly the Jesus of the gospels is ascribed this prediction (Mark 8:31; 9:31). Scholars debate the authenticity of this: see for example this article. However, if this were indeed the case that he predicted this, here we'd have something that would have significantly influenced the disciples' expectations and perhaps experiences in the wake of his death.

3) Though where might Jesus have gotten the idea of his resurrection after three days in the first place? Did it have any meaningful historical precedent, etc.? This is actually a pretty huge topic. Just to briefly point out one relevant Biblical text, though: Mark 6:16 suggests the unusual belief, ascribed to an unnamed large population, that Jesus himself was something like the reincarnation of John the Baptist, who had been beheaded by Herod. The crowd here wondered if "John the Baptist has been raised from the dead," and if in fact this was how "these miraculous powers are at work in [Jesus]" in the first place! (For my own part, I've actually argued at length that the historical Jesus indeed proclaimed some major supernatural event having to do with the miraculous destruction and rebuilding/raising of the Jerusalem temple in "three days," which may have relevance to the "raised after three days" death aspect.)

4) Was the catalyst — or at least a major catalyst — for the belief in Jesus' resurrection the mundane fact of a sort of misidentification of Jesus' tomb? That is, if some of his followers went to visit his tomb in the days after his death, as the gospels suggest, is it possible that they simply visited the wrong tomb — one that was unoccupied, and which then somehow led them to believe that his body had been assumed up into heaven? As you might imagine by now, Dale Allison discusses this in detail, but doesn't give it much credence,

5) The gospel of Matthew uniquely expresses this idea that Jesus' Jewish opponents actually expected the disciples to "stage" the resurrection, to make it conform to Jesus' alleged prediction while he was alive. Matthew 27:62-64 reads

62 The next day, that is, after the day of Preparation, the chief priests and the Pharisees gathered before Pilate 63 and said, “Sir, we remember what that impostor said while he was still alive, ‘After three days I will rise again.’ 64 Therefore command the tomb to be made secure until the third day; otherwise his disciples may go and steal him away, and tell the people, ‘He has been raised from the dead,’ and the last deception would be worse than the first.”

However, the historicity of this (along with the subsequent narrative of the placement of the guard to try to ensure that the disciples didn't do so) is more or less unanimously rejected. Because this is growing long, I'm going to be lazy and just link my comments here, which address scholarly debate over this.

Not a historian, just here to recommend r/AcademicBiblical for this question.

Are there any documents about Jesus from people who weren't Christians and didn't believe in him? Around that time period?

One thing that should be remembered is that Judaism and Christianity around the first century before and after CE is messy. In the popular framework it's simple, there was Judaism, then came Jesus and then the apostles spread Christianity after his death. Neat and simple. Actual history is a lot more complicated. There were a lot of smaller judaistic and gnostic sects with varying interpretation of the holy texts or even their own secret mysteries. And after the supposed appearance and death of Jesus there were very different opinions among the adherents about what Christianity was and the very nature of Jesus. Was he an angel? A created being? Equal to the Father or subordinate? It wasn't until after several councils that the basic tenets of the faith were agreed upon, the most famous being the Council of Nicea, which in 325 determined that Jesus was divine and co-eternal with the Father.

Great question, u/hnnng_booba, the same question was asked by a non-religious journalist who dedicated years of his life to finding the answer. He wrote a book about it and a movie came out too. "The Case of Christ" by Lee Strobel. I'd recommend reading the book, as it has a lot of interesting interviews he had with religious and non-religious historians, doctors, etc.

One bigger question I have to ride along with the OP.

Why would they (his followers or people of that time) want to make others believe he rose from the dead when there was no benefit? Historically speaking, it is true that people who did believe it (Paul, Peter, etc) ended up getting brutally murdered for it, and none of them gained any money or power for what they where doing.

So it is a big question I have as to why they where willing to give up their lives to the point of getting shitty deaths over it?