What are the empirical pieces of evidence at hand?
Exhibit A: Christian engages in storytelling, an oral tradition about Jesus
Exhibit B: At that time around 66-70AD, some eyewitnesses on the life of Jesus were still alive to tell the tales.
Exhibit C: There was a person named Mark who lived between 5 – 68 AD at that time.
Based on the above empirical data, we can derive the following cases of possibility interpretatively.
Case 1: That because of this oral tradition, one man of unknown identity- could be Mark or not, decided to write the story which he did not really witness first hand, this writing was called the Gospel of Mark. This postulated theory can be in-depth in the field of paleography study.
Case 2: One of the people (Mark) who witness first hand the life of Jesus contributed through His account the chain of storytelling about the life of Jesus and later wrote it without crediting himself as the writer although, external evidence can show that it was his writing. This postulated theory too can go in-depth under the field of paleography study.
Notice that based on the empirical data, it produces two opposite theories that contradicted from each other suggesting a further investigation to see what really happened, the two possibilities cannot be both true. The question becomes, what empirical data do we need so that we can definitely know which theory is true?
To allege that it was not Mark or that it was Mark, we have to have a basis of comparison between the gospel of Mark and Mark’s any original writing specimen and that is what we need, an original specimen of Mark’s writing so that an expert can study it using paleography, then whatever the outcome, it can be compared with the Gospel of Mark to see the difference. Here, the result will either confirm cases 1 or 2 empirically. But, the question is, do we have that evidence?
And so for now, we are stuck with 2 postulated opposing theories obtained from the empirical data, neither can be proven empirically which comes down to a choice of what to believe, either Case 1 or Case 2. Whatever your choice, it does not really matter but, be aware that regardless of your choice, it’s not empirically proven thus, a BELIEF.