Weapons
The ultimate defense of a Just Society is a well-armed populus. No mortal system, no matter how well constructed, can resist the evil of corruption forever. Subversion is inevitable. Written rules can be changed, distorted, or ignored to serve the wicked. All that is required is corrupt individuals bypassing checks and balances.
Laws are only as strong as the paper they’re written on.
If the government is working as it should, the citizens will see its laws as just. They will comply with the law to the best of their ability of their own accord, and when breeches occur, they will take responsibility for their actions as prescribed by law. Force is only required against the inevitable evil people within a society, and the good will support the government against them as needed.
However, in practice there will always be some level of corruption within the government because evil people cannot be kept out on a large scale. While the government will attempt to bring them to justice, no process can ever be perfect so they will persist and cause damage. In this case, weapons provide an important check against severe abuses of power. If an evil judge or government official crushes a good man and destroys his life, he has the means to fight back and get justice for himself, even if killing the petty tyrant is illegal.
A good man with nothing to live for can find something to die for.
When corruption becomes more extreme, the dynamic shifts from individual people who need to be brought to justice, to entire organizations. In this case, one man may not be able to do much on his own, but groups of well-armed citizens can. Organizations are made of people, and those people can be killed. Killing one or two may not accomplish much on its own, but many of the people in these corrupt organizations are cowards who will flee once a significant number of their co-conspirators are brought to justice.
While organized militia operations are the most efficient way to purge corruption and will tend to minimize the loss of good men, lone actors should not be underestimated. An individual can almost always guarantee a kill, especially if he’s willing to die to see justice done, and his example can inspire others. Men who are similarly oppressed will be inspired to follow the martyr’s lead, and thanks to the far greater number of good men being persecuted, they will quickly collapse the corrupt system through simple blood math. Their martyrdom can also inspire militias to form and act, leading to a more organized and effective movement to break the corrupt system.
Armed citizens can overcome an oppressive government.
Thus, the only guaranteed defense against this evil is citizens willing and able to fight back, and that requires weapons. This is why a Just Society protects the right of its citizens to keep and bear arms, and why the most useful definition of a tyrannical government is a government that disarms its people. The people in a Just Society must be well armed against the day the government is corrupted, because no organization made of fallible humans can resist corruption forever. The citizens will need weapons of war to overcome their oppressors in addition to their piety, so a Just Society must encourage them to own and become proficient with the weapons of the day.
Evil cannot be defeated if the good are defenseless, and those incapable of violence are helpless, not peaceful. Only those able to fight for what is right can truly be called God’s soldiers, and only they can build a Just Society in His image.