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 (1a) actually suggests a couple of specific applicable questions/criteria:


(SAC1) Was the author a Christian believer?
(SAC2) Was one of the main purposes of the author to promote Christian beliefs?


Why are these questions relevant to assessing the historical reliability of an historical document/source about Jesus?  The concern here is about objectivity and bias.  The point of view of an author can and usually does introduce bias into an account of events, and the purpose of the author in creating the document also relates to the possibility and likelihood of bias in the account.  


If a primary purpose of an author of an account of the life of Jesus is to promote Christian beliefs, such as the belief that Jesus was the divine Son of God, and the Savior of humankind, then that purpose introduces certain biases into the account.  Such an account will tend to select events that support this point of view and tend to exclude or downplay events that undermine or dis-confirm that point of view.  An author with such biases will tend to accept without question stories and details that support these beliefs, while tending to reject without good reason other stories and details that dis-confirm or cast doubt on those beliefs. 


Two general questions related to the above two questions would be:


(GQ1) What assumptions and beliefs does the author have about Jesus, and how might those beliefs introduce bias into the historical document or account of events?


(GQ2) Is one of the main purposes of the historical document or account of events to promote certain beliefs about Jesus or beliefs concerning some of the (alleged) teachings of Jesus?


Very general questions related to the general questions would be:


(VGQ1) What sort of ideological or philosophical or theological beliefs does the author have, and how might those beliefs introduce bias into the historical document or account of events?


(VGQ2) Is one of the main purposes of the historical document or account of events to promote certain ideological or philosophical or theological beliefs?


These fairly general questions relate to the categories of objectivity and bias.


CONCLUSION:
In evaluating the objectivity or bias of an account of events, one should consider (among other things) questions about the beliefs and assumptions of the author related to the subject matter of the account, as well as the purposes of the author in writing the account.