This is an excerpt from Part 17 of my Argument Against the Resurrection of Jesus posts:
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Doubting the Doubting Thomas Story

Why doubt the Doubting Thomas story? There are three different kinds of considerations that support a skeptical view of the Doubting Thomas story.

1. General Problems with the Gospels – including the Fourth Gospel
a. It was written by a Christian believer with the purpose of promoting Christian beliefs.
b. It was probably not written by an eyewitness.
c. It was composed decades after the crucifixion of Jesus.
d. It provides no attribution of specific stories or details to named and known eyewitnesses or sources.
e. It was written in Greek rather than Aramaic (the language Jesus and his disciples used).
f. It appears that the words and sayings of Jesus were preserved in oral traditions that failed to reliably preserve the original situations or contexts of those words and sayings, thus opening the door to misunderstanding, distortion, and corruption of the original meaning of Jesus’ words and teachings.
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Point (1b) suggests a specific applicable question/criterion:

(SAC3) Was the author of the account an eyewitness to the events in the life of Jesus contained in the account?

Why is this question relevant to an evaluation of the historical reliability of a Gospel or any other  account of events?  It is relevant because being an eyewitness to an event is a good way of obtaining accurate and detailed information about an event.  An eyewitness to an event is (in general) in a good position to know lots of relevant details about that event.  So, this question relates to the credibility of the author concerning the events that are described in the account.

Point (1d) suggests a related specific applicable question/criterion:

(SAC4) Did the author obtain information about the events in the life of Jesus in the account directly from one or more alleged eyewitnesses to those events?

A general question related to these specific applicable criteria  would be:

(GQ3)  How did the author of the account obtain the information about the events in the life of Jesus contained in the account? 

One good way to obtain information about an event is by being an eyewitness to the event, but there are other good ways to obtain such information.  Another would be to carefully interview an eyewitness shortly after the event occurred (like a newspaper reporter or a detective would interview an eyewitness to an event or crime).

It is important to recognize that eyewitness accounts are not completely reliable.  Physical evidence, such as fingerprints and DNA evidence, is becoming more important in criminal trails, because it has become increasingly clear that many innocent persons have been convicted of serious crimes, such as rape and murder, on the basis of erroneous eyewitness testimony.  This is also not merely a matter of dishonesty or deception on the part of alleged eyewitnesses, it is also a matter of the unreliability and corruptability of human memory.

Some very general questions related to the above specific and general questions  and to the credibility of the author would be:

(VGQ3)  How did the author of the account obtain the information about the events in question? 
(VGQ4) How reliable in general is that way of obtaining information about events?
(VGQ5) How reliable is that way of obtaining information for events or details of the type in question?

CONCLUSION:
The credibility of an author of an account depends heavily upon how the author obtained the information about the events contained in the account.  Ideally, the author would be an eyewitness to all of the events in the account.  But there are other generally reliable ways of obtaining information about events, such as carefully interviewing one or more eyewitnesses of an event.