When in reality it is very hard to know truth, even in the present time, even in one's own experience.
So, if that is the case, why make such an outlandish claim?
Well, there is value to someone if they can say 'I have the truth' and other people believe them.
The initial lie, once believed, takes on a life of its own thanks to the belief that becomes invested in it, especially when it is under someone's control externally.
>> Take the story of a group of fallen angels who descend to earth to seduce beautiful women. Using his new technique, Langlois discovered that earlier scholars had gotten the names of some of the angels wrong, and so had not realized the names were derived from Canaanite gods worshipped in the second millennium B.C.—a clear example of the way scriptural authors integrated elements of the cultures that surrounded them into their theologies.
The priests from the new religion wrote a text portraying the ancient gods as "evil". Sounds like an efficient use of propaganda to me. And that is something that makes historical research of religious texts very worthwhile.
For example, early Canaanite texts assigned territory to tribes according to their number of gods. In a later text, it was by their number of tutelary angels. In current texts it is unspecified why they got their allocation.
Oldest Canaanite texts have a goddess Asherah, equal to YHWH in importance (and always shown with a cake), involved in important rites. Nowadays they use a stick in her place, which appears to suffice. They clearly would have preferred to do away with her entirely, but weren't willing to abandon the rites that needed her.
Religious rites were indistinguishable from magic spells, and the ancients didn't care who they came from, if they seemed to work. They were swapped around like coleslaw recipes. You can see how they were thought of particularly in Numbers, where it clearly doesn't matter who uses them: if you say it, it works, priest or no.
For most of the known text we have a pretty good idea about where the earliest written copy comes from and serious academic is careful about ascribing truth to anonymous late 1st and 2nd century greek texts written in modern day Turkey and describing events happening decades earlier.
Christian theologians and biblical scholars however kind of have to take 2nd century anonymous texts from people who could not have been direct witnesses to the events described as gospel value because without those accounts there is no new testament.
Likewise, scholars generally agree that at least two of the gospels copied parts wholesale out of another (generally that Mathew and Luke copied from Mark and changed elements of his story). See the synoptic problem for more information about this. And that after they were written, others came along and changed or added elements. The ending of the gospel of Mark, for instance, is generally thought to have been a later addition, and at least some prominent scholars think the nativity story in Luke is. But again, we don't use the term "forgery."
For a more recent example, you don't see the term used to describe The Book of Mormon, which is purported to have been written two millennia before non-Mormons think it was written.
I suppose the term isn't used when matters of faith are the focus. The article uses the term to describe items created with the secular purpose of earning more money, even though they're religious in nature.
I think there are some good reasons why we don't call them "forgeries".
One is that people in the ancient world had different understandings of authorship from what people do today. Today, if you are copying a text written by someone else, either you copy it verbatim, or you very clearly mark your changes, so no one mistakenly thinks they were the work of the original author. In ancient times, people didn't necessarily think there was anything wrong with "improving" a text while copying it, and not failing to note the changes.
Nowadays, it sometimes happens that an author dies with a work unfinished, and a close friend or colleague is called upon to finish it off. However, we'll make it very explicit that's what going on, and to fail to do so is seen as unethical. In the ancient world, it was quite common for students to add stuff to texts written by their late teachers, based on their oral recollections of what the teacher said – or even just their opinions on what the teacher would have said – and few thought it necessary to explicitly note that they were doing it.
As these particular texts came to be seen as holy, intentionally changing them began to be seen as sinful. However, there was likely a lengthy period – possibly measured in decades – between the first drafts, and when they came to be seen as too holy to intentionally alter any further.
Finally, "forgery" generally implies something done to be intentionally deceptive, and we don't have any evidence that the authors of these texts were intentionally deceiving anyone. Even if they wrote falsehoods, for all we know, they honestly-yet-mistakenly believed they were true.
> For a more recent example, you don't see the term used to describe The Book of Mormon, which is purported to have been written two millennia before non-Mormons think it was written.
Part of the problem with calling it a "forgery", is that could be interpreted as making an assumption about Joseph Smith's psychology – that he was a conscious charlatan. While non-Mormons aren't going to believe he was a genuine prophet, genuine prophet and conscious charlatan aren't the only two options – is it possible that he was deluded? That he honestly, however falsely, believed that God was revealing stuff to him? Even if he was "making stuff up", was he consciously aware he was doing it, or was it unconscious creativity, which he subjectively experienced as coming from outside of himself, even though in truth it didn't? One of the reasons why scholars don't call it a "forgery", is it allows them to avoid that debate except when they specifically want to delve into it.
So is a text from, say, 200 B.C. that changes some things from an older version from 300 B.C. a forgery? Or is it just a different version of the same story and is neither more authentic than the other (the 300 B.C. version may not be the true original either)?
It seems to me that something is only a forgery if, say, someone in the year 500 invents an entire new text whole-cloth and then tries to pretend it's actually from 200 B.C. But if it originates from the same culture and traditions as the Hebrew bible then it's one version of an authentic text.