Historian explains why Jesus ''mythers'' aren't taken seriously by most Historians

[TheMessiah]

They're actually pretty good as historical sources, for gaining knowledge into the context of the time.

Like what?  The census of Augustus which never happened?

Please explain what historical "facts" can be gleaned from the gospels that are so important in your eyes?

I never said ''historical facts'' can be gained from it - but is it important to understanding the context of the time? Absolutely.

This post articulates it better than I can myself.

That said, the Bible was written by ancient people, and reflects a historical point of view. The tricky part with any ancient source is whether or not it can be corroborated with any other evidence. We have to consider the same issues when we deal with Herodotus, or Thucydides, or Caesar, or any of the other ancients who left texts describing their circumstances. We have to consider what the purpose is of this text. This is something that many people misunderstand about the Bible. They take something like Genesis and the Creation story, and say that since there is so much evidence for evolution, one cannot trust the Bible to report history accurately and it is all made up, when a proper understanding of genre (the purpose of any written text) will show that Genesis (especially the first few chapters) are much more in the wisdom literature and poetry tradition, and not history.
Many parts of the Bible were never written with history in mind. Some, however, were written with an eye to actual events. For example, there is archaeological evidence for King David's existence, as well as several other kings including Ahab[1] and Jehu[2] . Similarly, many of the non-Israelite characters in the Bible have been historically corroborated, such as Nebuchadnezzar[3] , Cyrus[4] , the pharaoh Shishak[5] , king Mesha of Moab[6] , Ben Hadad of Aram[7] , and many others. Therefore, The Bible is capable of providing additional insight into the happenings of the world in which it took place. However, this insight must be understood to be coming from the ancient Israelite perspective. If you don't trust texts by other ancient nations (such as the Epic of Gilgamesh, the histories of Herodotus, the Iliad and Odyssey, etc) without archaeological or other textual corroboration (such as two accounts of a battle taking place, etc), then you have some methodological difficulties.
This book is excellent at taking what we have in the Old Testament and comparing it with ancient sources and archaeology:
Grabbe, Lester L. Ancient Israel: What Do We Know and How Do We Know It?London: T & T Clark, 2007[8]
You ask what defines something as a good source. All of them are good sources, you just have to remember that all are biased, and our understanding of the sources corresponds with our understanding of the biases as well as archaeological confirmation of elements of the story.
For example, we have Ahab king of Israel, famously a "bad guy" in the Bible. In the Bible we see him interacting with Ben-Hadad and Hazael of Aram. In extrabiblical texts, we seen that Ahab was actually a fairly powerful king, contributing 2000 chariots and 10000 men to the Battle of Qarqar, a number probably made possible due to his earlier conquests of Moab and other nearby small countries. These earlier conquests are attested to by the Mesha Stele, written by Mesha of Moab and describing how they threw off Israelite rule after Ahab's death. Therefore, much of what is written in Kings and Chronicles (especially the later parts) is often taken to be historical, because enough of it has been corroborated by records from neighboring countries. Other parts of the Bible are understood to be more metaphorical, such as Noah's Flood. There is no geological evidence for a worldwide flood, as one might understand it to be described in Genesis 7. However, there are numerous other accounts of a major flood from other Near Eastern texts, such as the Epic of Gilgamesh[9] , the Sumerian King List[10] .
There has been evidence of flooding recovered around the Jemdet Nasr period (3000-2900 BC) and extending as far north as Kish. Also, there was evidence of major flooding uncovered by Robert Ballard in the Black Sea area[11] through the Bosporus Strait in 5600 BC. So there is evidence of regional flooding happening occasionally, but nothing on the global level. Therefore we see aspects of mythology and metaphor in that story, and it is not considered to be a reliable "historical" account of something happening.
So to sum up, parts of the Bible were written with the intention of recording historical happenings, these are mostly considered to be the "historical" books, and where they mention things that can be corroborated in other sources, they are corroborated. Many other parts of the Bible were not written from a historical perspective, but from a theological one, and therefore we use those texts in the same way we use other texts from the same period, to understand the people who wrote them, rather than understand the historical happenings of the period.
Does this make sense? It's sometimes hard to see what I've written in this tiny box. If you have any questions, I would be more than happy to answer them.