Um, not to be a dick, but couldn't it just be that since the Spear of Destiny has divine blood on it that it can pierce the Spectre as well being as how Jesus=God=Spectre. It doesn't have to be especially powerful if it's purpose is to simply slay/injure deities.
Example of my point being that Holy Water will destroy Demons but if you threw it on Luke Cage he'll just slap you silly with no effect.
I'm just saying...
It is a Spear so getting stabbed would likely hurt, but it's supernatural abilities might not be exactly universal in application. You might not be able to stab just anything with it with the same effects.