Rachels claims that Theological Voluntarism makes morality mysterious. What does he mean by this?

It is difficult to discern whether individuals can be designated as having good morals when an action is done not because it was moral, but because God has commanded it.

Explanation:

The idea of Theological Volunteerism is that an action has moral value because the action is approved or even recommended by God, through his teachings. In this context, what we as humans might consider as something that is morally right, may be considered in opposition to God's will or even deemed as irrelevant since it was not taught by God of certain religions as a morally right action that their follower should take.

A great example of this would be the idea of slaughtering some farm animals under the name of God for it to be consumed by the followers of the religion, even though it means that the method used to kill these animals is more inhumane. To the followers of the religion, this action is morally right since God has commanded it to be so; to those who aren't followers of the religion, it is a horrific and morally wrong action.