"Without justice, what are kingdoms but great robber bands? What are robber bands but small kingdoms?"

Yes, St. Augustine included the qualifier "without justice," but what State on earth is bound by justice? The State owes its very existence to funds (i.e., taxes) extracted under threats of fine and imprisonment. It operates like the Mob, raising its revenue under threats of great bodily injury up to and including death. If you don't believe me, try not paying your taxes for a while and see what happens. The State will kill you, in the final analysis, if you resist its fines and imprisonment.

Taxation is extortion. Extortion violates the Seventh Commandment. Until we have a State that raises its revenue voluntarily--i.e., until we have a State that operates like a church or legitimate business, relying on charity or commercial exchange--I'd say we Christians are under no moral obligation to obey its generally arbitrary and capricious edicts.

Of course, the State can ruin your life in this world, so be careful. But the care you exercise is entirely a prudential matter. It has nothing to do with upholding or rejecting Christian morality.