Lance of Longinus

The Spear of Longinus (神様殺しの槍 (ロンギヌスのやり) ) is a legendary weapon used by to pierce the side of the Son of God during his crucifixion, and thus killing him.

Etymology
The kanji used for the spear is called God Killing Spear, which refers to it being the weapon that killed the Son of God instead of his crucifixion.

Principles
The spear was derived from the passages of the Bible that depicted the Son of God being pierced by the weapon on his side by a Roman soldier to check if he was dead. In so doing, they could determine if they need to break his legs in order to hasten his death. After being pierced, blood and then water came from his side, signifying that he was indeed both man and divine.

The name Longinus came only later in Christian traditions, and is also venerated by many Christian denominations.

Longinus Replica
It is unknown what powers the true Holy Spear has, but for the mass produced ones used by the Thirteen Knights of the Roman Catholic Church, known as Longinus Replica (量産聖槍 (ロンギヌス＝レプリカ) ) it can be conjectured. If one applies Idol Theory, one could assume that it is able to harm or even outright kill Saints as their bodies are similar to the Son of God, probably on par to the Croce di Pietro.

However, according to Index, holy relics like the Spear of Longinus could never truly be substituted, so one can assume that a replica's power does not equate to that of the original.

Index Arc
Index uses the spear as an example on how strong her Walking Church is. Later, Index mentions the lance, among other important relics of Christian tradition, as something that cannot be substituted and shows the limits of magicians to make substitutes of relics being used in rituals.

Deep Blood Arc
The Holy Spear, as it is also known, is also referred during the arc, where the Thirteen Knights of the Roman Catholic Church is said to be trying to mass produce them.

It is also mentioned later by Aureolus Dummy, comparing Touma's Imagine Breaker (which Touma often stated that could kill God), to the God Killing Spear, for effectively negating his Limen Magna.

World War III Arc
During the Battle of Dover Strait, when facing the Maiden of Versailles wielding Durandal and using shards of the Curtana Second to form her own sword, Carissa refers to how Charlemagne inserted a shard of the holy spear into.

Toaru Majutsu no Index SS: Kanzaki
Brunhild Eiktobel briefly states to Kanzaki Kaori that her saint side would interfere with the creation of the replica Gungnir changing it to be modeled after the lance instead of the it's true function.