It's authentic yes, but that isn't the issue. The issue is whether or not speculation is correct that it IS the spear of Longinus. It doesn't say what exactly it is in the book except that it was a knife. And knifes aren't spears exactly, only if its the tip of said spear.
This is true. The vault was Vault 7, housing artifacts of "western religious significance" and the contents were all described as christian directly, or were general art from periods dominated by christian imagery. The pedestal contained 5 artifacts, only two of which are explicitly identified (Grail and Shroud); the Crown of Thorns is kinda obvious though. That's 3/5 of the artifacts being Crusafixion-related, with the placard easily fitting as well (but being an extremely generic object so its hard to confirm without knowing the writing on it). The Spear is the only bladed object I know of related to that field, but it could be something else with a connection not part of the popular myth (something specific to the DV perhaps).
That all being said, my entirely doylistic logic is that when Jim took a Bladesmithing workshop a few years back his brain created this thing, which sure looks like how I picture the thing in Hades
Curator's edit: Here's
the link to the image from the original website. Please do not post it as an image on the forums, as that is bad etiquette without hosting it somewhere first.
And here is the main page of that website -Serack