If a system cannot legislate morality, that system is doomed to anarchy. Without right and wrong, there is no need for justice.
Morality distinguishes right from wrong in a specific context.
Morality requires a context. It itself is not one. As such, a system must define a context or succumb to systemic decay and end in anarchy.
Modernism attempts to define its context a posteriori, but morality requires a margin of a priori context. Without external intervention such as a command architecture (as in communism) or propaganda, such a system will decay toward anarchy.
Relativism attempts to employ all contexts. A system can employ manifold contexts, but conflicts must be resolved by a more general context. Relativism presumes that no general context exists to resolve conflicts. Therefore, without external intervention, such a system will decay toward anarchy.
Theism suggests a single ultimate context for morality. Such a context allows a system to avoid decay.