The Separation of Church and State

We can divide efforts for the common good into two big categories: attempts to make yourself promote the common good and attempts to make everyone else promote the common good.

Succeeding in the second goal (making everyone else behave) is clearly much more effective than succeeding in the first, assuming you're not the only person left on the planet. For example, if you want to establish a welfare system, you're going to want a tax law that makes everyone chip in to support the poor.

On the other hand, attempts to make everyone else behave can be counterproductive. You might think it is good for individuals to live on meager sums and donate all their other earnings to good causes. But the economy would probably collapse (or at least wind down to a communist grind) if you passed a law that makes everyone do this, since most people work only enough to earn the money that they are allowed to spend on themselves. Such a law would essentially be a mandate for communism, and communist societies tend to have even higher rates of poverty.

The counter-productivity (literally, in the above example) of trying to make everyone else behave is precisely what gives rise to the need for separation between church and state. When governments go too far in forcing individuals to promote the common good (or "God's good"), the common good suffers.

It is not always clear where to draw the line. Should the government legalize cocaine? Refraining from abusing drugs is beneficial for individuals, but enforcing laws that require everybody to refrain from doing cocaine often requires violent measures or long prison terms.