OK, this is a recurring subject on this subreddit, as noted there are a few FAQ answers featuring /u/talondearg, /u/jasoncaspian and /u/kookingpot. I'm going to try and adress your questions directly.

A theory put forth by a minority of scholars known as the Christ myth theory

"A minority of scholars" is putting it euphemistically. Of prominent mythicists, I'm not sure if there's anyone except Robert M. Price who could be regarded as a serious scholar - and as far as I know, his position is quite a bit more nuanced than that of e.g. Doherty.

but I don’t see why this doesn’t hold water because the only documents that talk about his life are Matthew mark luke and John which are religious documents.

Hold your horses! First, any serious Christ Myth theory has to propose an alternative explanation for the origin of early writings mentioning Jesus. Merely critcizing the gospels on a supposed lack of integrity doesn't make a myth theory more plausible than a historical Jesus.

Second, there are extrabiblical accounts of Jesus. The most important by far is that of Josephus' Testimonium Flavium. The TF has been compromised, possibly by the incorporation of margin notes into the main text which exalt Jesus, but a later mention of James (Jacob), the brother of "Jesus, who was called the Christ", makes it very unlikely that the TF was simply invented.

Unlike other religious figures such as Muhammad and Buddha, Jesus himself did not write any books and the gospels while they seem like primary sources since Jesus is speaking in first person, they are not. If we applied this standard to any other historical figure, we would not know much about them

I'm not well-versed in the early sources of Buddhism, but Gautama Buddha lived some 150 years before the introduction of a writing system in Sanskrit, so his teachings had to be transmitted orally before then - much like the sayings of Jesus were. There's a similar situation with Muhammad whose teachings are at least traditionally thought to have been compiled by the caliph Uthman around 650, some two decades after the prophet's death. A very extreme case of this is the Gathas of Zoroaster, which were transmitted for about 1500 years before being written down!

This situation is comparable to the sayings of Jesus being transmitted (and at some point written down) for a few decades after his death. If you read Mark with a critical eye, he often appears to be contriving narratives around quotes which he himself does not seem to comprehend, often trying to explain them after. I like this example from Mark 3:

22 And the teachers of the law who came down from Jerusalem said, “He is possessed by Beelzebul! By the prince of demons he is driving out demons.”

23 So Jesus called them over to him and began to speak to them in parables: “How can Satan drive out Satan? 24 If a kingdom is divided against itself, that kingdom cannot stand. 25 If a house is divided against itself, that house cannot stand. 26 And if Satan opposes himself and is divided, he cannot stand; his end has come. 27 In fact, no one can enter a strong man’s house without first tying him up. Then he can plunder the strong man’s house. 28 Truly I tell you, people can be forgiven all their sins and every slander they utter, 29 but whoever blasphemes against the Holy Spirit will never be forgiven; they are guilty of an eternal sin.”

30 He said this because they were saying, “He has an impure spirit.”

Mark's addendum, bolded, seems to be connected to the last part of the quote. Jesus' argument seems to be that Satan is waging a spiritual war on mankind by possessing people, and that performing exorcisms disguised as a holy man would work against his purpose. But the connection between this argument and Jesus' declaration of the "unforgivable sin" is not really immediately clear, to us or Mark; the leap one can make is that Jesus was filled with the Holy Spirit on his baptism and that he is warning them that they are not slandering the man Jesus, but the Holy Spirit, which Mark seems to be suggesting. It's possible that Jesus said the quote like this; it's also conceivable that 3:28-29 were originally a separate quote that at some point was copied as an addendum to 3:23-27. In either case, this is the kind of thing that suggests that there existed collections of quotes by Jesus which Mark had access to (along with documents like the Sayings Gospel of Thomas), and that the purpose of the Gospel was to compile these teachings, presumably along with anecdotes about Jesus' life, into an accessible tract written in the style of Greek biographies.

Matthew and Luke used mark as a source and a document that is lost known as the “Q” source. The gospel of John is completely different from Matthew mark and Luke and gives a different account of Jesus.

Matthew and Luke seem to have had other sources as well, conventionally called M and L; John, as you note, had other sources (there are various deconstructions of John that attempts to reconstruct them). It's not true that they are completely different, though, albeit there are many important theological distinctions. But the basic biographical details of Jesus are pretty consistent:

• He was a Jew from Nazareth (Luke notoriously contrives the story to have him born in Betlehem)

• He had a mother called Mary and a brother called James, along with other siblings.

• He was a simple craftsman (tektwn) by profession (Mark), or at least the son of one (e.g. Matthew) (but note also that son of x is a poetic way of saying x in Aramaic tradition).

• He travelled around preaching with a group of followers called "the Twelve".

• In Jerusalem, he attracted crowds, and with them, the attention of authorities.

• He was betrayed by Judas Iscariot, one of his disciples, and executed by Pontius Pilate.

• On the third day after his execution, he was thought to have risen from the dead, with Paul suggesting that James had a vision of him.

I’d like to add that the Gospels aren’t even the earliest documents that talk about Jesus’s life and describe Jesus as a “real” person; Paul’s undisputed letters do so, and they all predate the Gospels (assuming the scholarly consensus for dating the Gospels is accurate). Simon Gathercole wrote an article recently, The Historical and Human Existence of Jesus in Paul's Letters published in the Journal for the Study of the Historical Jesus, arguing that Paul’s undisputed letters provide much more evidence as to Jesus’s humanity and historical activity than is commonly assumed. I will paraphrase and/or quote the relevant facts that Gathercole believes the undisputed letters provide some evidence for and provide the cites he uses where available.

-Gal. 4.4: Jesus was born of a woman

-Rom 9.5, Gal. 4.4: Jesus was a Jew

-Gal. 3.16: Jesus is a descendant of Abraham

-Rom 1.3: Jesus is a descendant of David

-Gal. 3.19: "Jesus, as the seed of Abraham, came"

-Jesus’s earthly activity took place in early to mid-Herodian era.

-Jesus is designated as a man by the letters

-The name given to Jesus (i.e., “Jesus”) is a human (as opposed to angelic) and Jewish name

-Rom. 8.3, 1 Cor. 11.23-25, et al.: Jesus had a flesh and blood body

-1 Cor. 15:5-7: It is strongly implied that Jesus was known during his lifetime to Cephas, James, a defined group of 12, certain apostles, and 500 others

-1 Cor. 15.7: Jesus had brothers who missionized in his name

-Gal. 1.19: One of the brothers was named James

-1 Cor. 9.5: Cephas and Jesus' brothers were married, the context of which implies that Jesus was not married

-1 Cor. 15.6: Jesus may have had a following of at least 500 people in his lifetime

-1 Cor. 7.10: Jesus "appears to have had a zero-tolerance attitude to divorce"

-1 Cor. 9.14: Jesus taught that workers should be paid

-1 Thess 4.15-17: Jesus provided eschatological instruction

-Rom. 5.18-19, Phil. 2.8: Jesus is obedient to God

2 Cor. 10.1: Jesus "was known as 'meek' and 'gentle'"

-Gal. 2.20: "Love is evidently a feature of Jesus' willingness to die for others."

-2 Cor. 5.21: Jesus did not sin

-2 Cor. 1.19-20: Jesus was faithful

-Gal. 6.17: Jesus suffered wounds

-Rom. 15:3: Jesus suffered verbal abuse

-1 Thess. 2.14-16: It is implied that Jesus “was disbelieved by majority of, or the most powerful, Jews during his ministry"

-1 Cor. 11.23-25: The night prior to his death, Jesus instructed an audience to repeat the breaking of bread and drinking of wine in remembrance of him

-1 Thess. 2.14-15: Judaeans instigated Jesus’s death

-1-2 Cor., Gal., Phil.: Jesus was executed via crucifixion

-1 Cor. 2.8: Jesus was condemned to crucifixion by the "rulers of this age," most likely earthly authorities (implied with 1 Thess. 2.14-15 to be Jerusalem elites)

-Gal. 1.4, 2.20; Phil. 2.8: Jesus went to his death willingly

-1 Cor. 15.4, cf. Rom 6.4: Jesus was buried

This isn’t to say that all of Paul’s assertions are necessarily true (and Gathercole is not arguing as such), but taken together they do strongly suggest that Jesus’s earliest followers believed that he was a “real” person.

There’s an FAQ section dedicated to this: https://www.reddit.com/r/AskHistorians/wiki/religion?utm_source=share&utm_medium=ios_app#wiki_did_jesus_exist.3F