If theists understood "evidence"

[Jehanne]

But, you act like the early Church(es) unanimously considered the Gospel of Matthew to be divinely inspired, and again, such was not how the New Testament came to be:

Wikipedia -- Development of the New Testament canon

Yes, I am aware, that the declaration of Canon was not perhaps as authoritative and clear cut as some might like.  I also no some who use information like this selectively and incompletly to suggest things that are not true.   Kind of like trying to surprise people that the manuscripts are filled with errors, but suggesting in silence something other than what the data indicates.

It is interesting that you bring up Marconianism, referred to early in the Church as being unorthodox, and given a fair amount of credit for the start of the focus on Apostolic succession by Irenaeus.  I understand that there where some groups (which we can see as isolated and having some growth in history) which did differ.  And it was the connection of the teaching of the apostles and those who knew Jesus, that was the answer which was traced back to; in order to resolve this.   But there where some who certainly had their own ideas which did not trace back to Christ.

You see this as well in Paul's letter to the Gallations;  which most scholars date back to the 40-50s or at latest 60's and which wikipedia states  has near universal consensus was written by the Apostle Paul (since I know such things of are great concern to you).   Even here, there are those who where trying to distort the Gospel for their own ends, and Paul gives a rather stiff warning that if they or another teach you any other Gospel, that they are  to be accursed. With this, I find it rather difficult to believe, given that we do have quite a few disputes in history over authorship and canon, that the authorship of the Gospels would go so unnoticed a century later in some conspiracy theory to attribute authorship to them. And then there is the question of why would they do this?   You don't see in the early writings, the challenges and discussions as to the historicity of these things, which largely arose in the 19th century.

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I am curious in your support for the claims you made which you make from above though. And what gives you such force to use the highlighted words.

1. )  We can be confident that the work Papias was citing was not the gospel of Matthew that we have today
2.)  an anonymous gospel which likely drew from earlier sources attributed to Matthew existed by the latter half of the second century
3. ) There appears strong reason to believe that neither were written by eye witnesses.

I would like to know the reasons for this confidence in these claims.

I don't believe Paul, for reasons that I have stated before, namely, I believe that Paul was a "storyteller" who told overt lies, in particular, his claim to have persecuted early Christians.  I think that this story was invented by Paul in order to give him credibility within the early Christian community, but within the known structure of the Roman Empire in Palestine, I think that his claims were impossible.  Neither the local nor the Roman authorities would have tolerated such behavior, either due to one of their citizens or one of their subjects.  Paul's claims are equivalent to someone claiming that Sheriff Joe Shapiro went to New York City, set up his own jail in a rented hotel room and started making his own arrests.  A few decades from now someone may write such a story, and while it may make for entertaining reading, it would be viewed as being an historical impossibility.  Ditto for Paul.

By the way, there is a principle of law at work here:

falsus in uno, falsus in omnibus -- It means that when a defendant lies about one thing, it is proper to assume he lied about everything.

Paul is not a credible witness to history -- he likely suffered from epileptic seizures, which he interpreted as being of divine origin, he believed in a flat Earth, and he was a liar.