Money and justice do not mix.
That was Jesus' perspective when he said "you cannot serve God and mammon" (Matthew 6:24). The Aramaic word he used for money basically means "bribe."
The coin of the realm was Caesar's; like other Galileans, Jesus believed that God's kingdom would supplant Rome's empire. Roman rule for him was not a blessing, a civilizing influence, or even (as it featured in Paul's mind) an instrument of punitive justice. Military occupation was a fact of life, to be accepted only as part of what God desired to transform.
From a biblical point of view, it is in any case strange to think of government as admirable. "Put not your trust in princes" (Psalm 146:3) is a typical admonition, because the brokenness that is a part of human identity does not disappear when people organize into nations.
When Jesus said, "Caesar's repay to Caesar, and God's to God" (Mark 12:17), he certainly did not assume that Caesar was just. The context of those words makes this plain. Before his famous saying, Jesus demands, "Bring me a denarius so I can see," and then asks, "Whose is this image and inscription?"
Opponents had asked him whether or not to pay tax to Caesar at all. They wanted to maneuver Jesus into a position of refusing to pay the tax, since he was known not to favor the Romans. Such a position would amount to insurrection, and Rome executed anyone who promoted tax revolt. In one case of rebellion, just after the death of Herod the Great, the military commander ordered up to 2,000 crucifixions to display the imperial might.
Jesus counter-question about the image and inscription forced his opponents to acknowledge the coin as Caesar's property. It wasn't good, but it was Rome's. The teaching about what is God's and what is Caesar's assumes that, just as the Roman coin bears the image of the empire, so God's kingdom makes itself evident with its own stamp.
Jesus saw the primordial source of people, male and female, as the image of God; they bore the divine likeness (Genesis 1:27). (The same idea features in Jesus' teaching in regard to marriage, Mark 10:6.) Understanding that Caesar's power is limited to what Caesar can do -- mint coins in this case -- sets the boundary of political authority in comparison with humanity's fundamental connection to God. As long as people can keep their inherent identity intact, Caesar can have what is owed him, and only that.
Jesus' position explains why, for him, the right question about taxing is not who should pay, but: What should be taxed? And the answer is: mammon. What comes from Caesar can go back to him, involving God and his people as little as possible. For that reason, Jesus interfered in the Temple when monetary transactions were permitted there. When he drove out the moneychangers and merchants, he acted on principle, not momentary anger.
The latest attempt I am aware of in Congress to tax money rather than people would charge three cents for every hundred dollars in Wall Street trading. These days, of course, the interest is in deficit reduction, and an estimated revenue increase $352 billion over 10 years would obviously help.
But transactions do not only occur on Wall Street, and wherever and whenever they happen, government is necessarily involved in the provision and regulation of money (in all its forms). Experience has shown that taxing people according to their income leads to arcane computations of what income is, as well as sheltering on a gymnastic scale. Taxing them according to their ownership of land brings other distortions (that income tax was supposed to have rectified). A comprehensive transaction tax could not only address the current shortfall in government revenue, but also replace taxes that are so intrusive that they feel burdensome no matter what their level is.
Why exclude money from tax, even as it changes hands? Those who profit from many transactions benefit from government services that issue, control, and insure the value of monetary instruments. Yet many do not pay for those services, and some have expected to be bailed out by taxpayers when they make bad decisions. That pattern is a demoralizing inequity of globalization, and should come to an end.
People do not need to pay for the money they use; money can now pay for itself. Keeping mammon in its place sets limits on government, even as it keeps Caesar at arm's length by according him his due.