Okay, so at the end of Skin Game, Harry is in possession of several powerful, genuine religious artifacts particular to the Christian faith, all described by him as (and apparently confirmed by Hades to be) weapons.

I understood most of them, but one tripped me up as to what exactly it was, and I'm also curious as to what their purpose as weaponry could be.

The Holy Grail was lost, and even Michael wasn't sure how it could be used to do evil since it's significance was apparently wholly good in nature (ultimate expression of God's grief and love, etc.), but seeing as how Nicodemus used the (knock-off) Shroud of Turin to create and try to spread a super plague, I agree with Harry that Nicodemus will probably figure out something.

Then there was the Crown of Thorns. Dunno what the weaponized value is of that.

The placard, which I'm pretty sure is the one that was put around Christ's neck, saying that he was the King of the Jews? Again, not sure how that's a weapon.

But the one I'm totally confused about is the knife? I'm not religious and not terribly well-versed in the particulars of the Resurrection story, but the knife's significance must be pretty darn obvious because Harry immediately just knew that that was what Nicodemus wanted. What's it's significant in the story of Christ? I vaguely remember there being a spear that they used to cut into Christ's side after He died, but unless the "knife" is actually a spearhead... What's the deal?

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Curator's edit, Below are links to some of the responses I'll highlight.  ^-Serack

Quantus shows a plausible interpretation of what the "Knife" looks like
Quantus's excellent writeup on what the Relics could be
A quote of an alternate list by Phariah
Quantus's excellent post equating the Relics to some Egyptian concepts of the soul, and Eldest Gruff's excellent response.

Curator's edit 2:  I added some formatting to the OP ^-Serack

For additional discussion on the relics, see this topic