Consider this statement, which we can assume to be true.

"The maximum amount of force my spear can exert on you solely depends your armour."

Doesn't sound impressive yet?

Think about it more carefully, and you'll notice that this spear can actually pierce all armour. By Newton's third law, the spear can only exert a force on the wearer of magnitude equal to the wearer's reaction force.

If in a certain case, the spear cannot pierce through the armour, then it would only be able to exert the same force on an even tougher piece of armour. In order for the above statement to be true, the spear must be able to pierce through all armour.

Just a random thought.