The Holy Bible.
The same Bible in which Jesus cleanses the Temple by literally turning over tables and whipping the people who have turned God's House into a den of theives.
I am not suggesting we beat up Harold Camping or otherwise harm him, but after having to deal with people who are either convinced that what he was saying was true, and after having to deal with non-believers who attempted to use this blasphemy to attack Christianity, I find the only way to deal with such people is to militantly and ferociously rebuke them... with Scriptural Truth, not physical violence.
I will never attack anyone for their belief system (though I will evangelize to them if they are curious... I know this is the coward's way out, but it is my sin that I keep committing), but if someone attacks my faith, I pray that I will have the guts to respond. After all that Jesus did and continues to do for me, every single day, and with my unworthiness of His love and grace, it is the very, very least I can do for Him.