The Gospels were all written at least 50 years after Christ (Mark) to 100 years after him (John).

We think the Gospels are inspired by works we have since lost. The most famous is the "Quelle" or Q source (Quelle means source in German), which is supposed to be a list of sayings of Jesus.

Basically what Bible scholars do is they read each text meticulously, line by line, then they compare what is similar and different between each. Doing this - it is pretty ingenious - they are able to reverse engineer earlier texts and even figure out what percentage of each text comes from each earlier work. We started doing this in the 1800s and it's persisted to present day Bible scholarship.

The other early work (other than Q) is Mark. Basically we think Matthew and Luke used Mark (the earliest Gospel) and Quelle as sources.

The first three Gospels are called "synoptic," meaning they have roughly the same presentation of the Jesus story.

The last Gospel, John, is considered anomalous so it gets its own category. It doesn't have the same sources as the others and Jesus goes on long, philosophical sermons in it which he doesn't in the others.

We call the authors of the Gospels the evangelists.

In the New Testament, Matthew was placed first because originally we thought it was the earliest Gospel. This conclusion came from its (mostly Jewish-oriented) perspective. More recently scholarship has accurately found Mark was the earliest, using the reverse engineering method I mentioned.

There is some division over how you can interpret each Gospel but I usually present it to my students as the following:

Matthew - The "Jewish" perspective on Jesus

Christianity was originally a sect of Judaism, so Matthew is often more concerned with interpreting Christ's teachings in relation to Jewish law and tradition. Matthew is also the only Gospel written in Aramaic, not Greek.

Luke - The "Gentile" (or Greek) perspective on Jesus

As time proceeded, Christianity began to attract adherents who were non-Jews (Gentiles) - mostly people from the Hellenic world - but there was much resistance to this as it was originally a Jewish sect. Luke's view is often focused on interpreting Christ to make it more accessible to non-Jews.

The next two are my views of the other Gospels and how I usually teach them (I like to keep it simple and clear), but just know you could dispute these and interpret them differently:

Mark - Emphasis on the "Humanity" of Jesus

As you know the Christian view of Christ says he has a dual nature, both human and divine. I base this reading mostly on the very interesting conclusion to Mark - the resurrection. The resurrection is left ambiguous in the original version of Mark. Jesus does not reappear to the disciples or overtly demonstrate his risen form. This Gospel also focuses on the "mystery" of who Jesus is. It is accepted he is a prophet, and he demonstrates miracles throughout, but it is unclear who he is.

John - Emphasis on the "Divinity" of Jesus

And this is how I usually frame John: as one in which the divinity of Christ is accepted - there is no "mystery" like in Mark. Focus on his godhood rather than his humanity. For example, he begins as a divine "Logos" which preexisted.

Getting back to your question - you raise a very interesting point about the unique situation in which there are four different accounts of effectively the same narrative in Christianity.

The Gospels do offer different versions of events, tweak the exact things said by Christ, and contradict each other at different points.

While in earlier epochs you might run into the danger of being accused of heresy, I feel it enriches the religion greatly and opens it up to many different analyses and lenses.

Perhaps it is also not the point we are supposed to get the exact version of what happened, but rather understand the overall synthesis or theme of Jesus' teaching.

The textual history of the Bible is quite complex and fascinating. The four Gospels can be divided into two groups: Synoptic gospels (Matthew, Mark, and Luke) and the Gospel of John, which is narratively and theologically distinct from the others.

Scholarly theories vary but one of the most common over the past 100 years or so is that Mark was the first Gospel (circa 60 AD), and Matthew and Luke were developed from Mark in combination with a lost source or sources, and John was the latest, from about 100 AD. This is very much a simplification of the distinctions between the four, but while all four gospels focus on biographical details of Jesus, the Synoptic gospels are more concrete, focusing on specific stories of Jesus's life, and John is much more abstract and philosophical in style.

Certainly there were others floating around; some have survived to this day (Gospel of Thomas, Gospel of Mary, etc.) and others have undoubtedly been lost over the centuries. During the first few centuries, there were a lot of arguments and differing philosophies about the developing Christian faith; the losing ideas ended up being deemed heretical. The books which eventually came to form the accepted canon of what we know now as "The New Testament" were developed by the end of the 4th century AD by various bishops who came up with the lists of gospels and letters which they felt were both accurate and - importantly - best reflected the theories of Christianity which matched the prevailing, orthodox(with-a-small-o) theology at the time. Primary sources which reflect the first modern New Testament Canon include St Athanasius's "39th Festal Letter" as well as the Codex Vaticanus, written in Rome about 30 years earlier.

As a preface, none of this is certain. Scholars are simply attempting to make their best guesses as to the answers to each of your questions based on the limited evidence available to them. Virtually every position below is disputed by at least one reputable scholar.

Mark, Matthew, and Luke are known as the Synoptic (lit. "seeing together") Gospels because their content is so similar that it can be set out in columns next to each other and read side-by-side. The academic consensus is that Mark was the first written and that Matthew and Luke adapted Mark. Matthew and Luke would have done that to produce Gospels that they believed to be "better" for their intended audiences than Mark (in addition to adding more material and focusing on different issues, Matthew and Luke both improved Mark's grammar). The most common position is that Luke and Matthew were independent of each other and that each was combining Mark, an unpreserved separate source known as "Q," and their own unique traditions. (Other theories exist, such as that Q never existed and that Luke simply had access to Matthew as well as Mark). Mark was probably written in the late sixties or early seventies. Matthew and Luke were likely written in the eighties or nineties.

John is an entirely different beast. Over 90% of John has no parallel in any of the synoptic gospels and it has an entirely distinct character from the Synoptic Gospels. Unlike Matthew and Luke, John shows no obvious dependence on Mark and the author of John may or may not have had access to it, Matthew, or Luke. Most scholars date John to the mid-nineties.

Mark is normally presumed to have written for his own local Christian community, although others have argued the Gospels (including Mark) were intended to instruct the Church at large). Arguments in favor of the former position include the use of personal names of specific individuals who are not otherwise referenced in the NT, that certain prophecies appear to be directed to specific locales, and that the audience of Mark appears to be persecuted (despite the absence of systemic, as opposed to local, persecution at the time Mark was written). Additionally, the continued survival of Mark despite the existence of Luke and Matthew may point to the local use of Mark. Traditionally, Mark was presumed to be writing from (and, under the standard presumption, to) Rome. However, Roman Syria may be more likely for geographical and demographic reasons. Mark's Gospel was also likely targeted primarily to those who were already Christian (rather than adversaries or nonbelievers); it presupposes Christian belief at numerous points and does not provide a reason for the disciples answering the call of Jesus (unlike in similar tales found in philosophical and wisdom literature). Mark did not presume that his audience would know Aramaic or many Jewish customs and laws (which, along with its status as a war-front, likely eliminates Palestine as its place of composition, despite a number of proponents of that theory). The traditional answer as to the purpose of the Gospel of Mark is to preserve the Jesus tradition. However, this requires two arguably false assumptions: (1) that those in Mark's time believed written traditions to be superior to oral traditions (2) that Mark did not himself rework the Jesus tradition in his own writings. Some scholars argue that Mark wrote to address a crisis in the early Church's eschatology caused by the Jewish war or to counter a Christological heresy. Others argue that Mark was written pastorally to remind readers and listeners of who Jesus was, what he came to do, what he would do, and what the Kingdom of God constituted. It may also be that Mark was intended for liturgical services.

The Gospel of Matthew was most likely written to Jewish Christians. Reasons for this position include: its stress on the fulfillment of the Old Testament and Jesus's righteousness and fidelity to the law, the omission of explanations of Jewish customs found in Mark, Matthew's decision to present a number of discussions in rabbinic patterns, and refutations of Jewish claims against Jesus (e.g., that his body was stolen or that his birth was illegitimate). Most scholars believe it was written in Syria (and most of those scholars believe it was written in Antioch). There is less certainty as to its purpose. It clearly recounts the life of Jesus, explains how Jesus fulfills the Old Testament, and proclaims the saving activity manifested by Jesus's death and resurrection. In doing so, Matthew likely intends to help his church or churches understand their new faith in the interim between the death and resurrection of Christ and his second coming. In this manner, Matthew acts as a discipling manual, a "handbook" of Jesus's life and teachings. Further suggested purposes include: to act as a midrash of Mark, to act as a lectionary or as a catechetical manual, to address divides within Matthew's community, to act as missionary propaganda, or to function as a polemic against rabbis or Pharisees.

Luke's Gospel contains a preface stating that he intends to write an orderly account of "the events that have been fulfilled among us" for Theophilus (a name meaning "friend of God" that may be a pseudonym), so that he "may know the truth concerning the things about which you have been instructed." The traditional assumption is that Luke wrote pastorally to a Gentile church or churches. However, he may have been writing to an audience of God-fearers (non-Jews of a Hellenistic background that are attracted to Judaism and follow some of its precepts but have not converted). It should be noted since Luke is explicitly addressed to Theophilus, it is possible, but not likely, that Luke was written specifically to address the needs of Theophilus and not a wider readership. Luke's ideal audience would have been of higher education than average, knowledgeable of northern Aegean Greek Culture, and familiar with the Septuagint.

The Gospel of John is widely assumed to have been written in either Roman Asia (likely Ephesus or Smyrna) or Syro-Palestine (likely Galilee or Antioch). Its own statement of purpose is that it was written "so that you may come to believe that Jesus is the Messiah, the Son of God, and that through believing you may have life in his name." However, uncertainty as to the underlying Greek text and ambiguity as to its meaning renders it unclear whether that statement is missionary or instructional (i.e., to outsiders or to members of the Church) in character. A majority of scholars believe the latter, while a minority believe it has a missionary or dual purpose. The Gospel of John was certainly written to provide an authoritative interpretation of the Jesus tradition. It is less clear whether John was written to supplement, correct, or interpret the synoptic Gospels (or even if John was aware of those Gospels). However, the Gospel of John does have definite polemical aims against contemporary understandings of John the Baptist and against "the Jews" (generally referring to Jewish leaders, especially Pharisees, who were opposed to Jesus and his followers, not the entirety of the Jewish people). It may also be anti-gnostic and anti-docetic, although this is far less certain. The majority of community to which the Gospel of John was addressed were likely Jewish Christians, the core of whom would possess the Palestinian Jewish cultural knowledge assumed by the Gospel.

Mark Allan Powell, Introducing the New Testament: A Historical, Literary, and Theological Survey (2d ed. 2018) Craig S. Keener, The Gospel of Matthew: A Social-Rhetorical Commentary (2009) 1 Craig S. Keener, The Gospel of John: A Commentary (2003) 1 Craig S. Keener, Acts: An Exegetical Commentary (2012)
Joel Marcus, Mark 1-8 (2000) George R. Beasley-Murray, John (rev. ed., 1999) John Nolland, Luke 1:1-9:20 (2000) Robert A. Guelich, Mark 1-8:26 (1989) Donald A. Hagner, Matthew 1-13 (2000) NRSV