An heteronomous ethics is one which is shaped by factors outside the agent rather than by the agent’s own sense of good and evil. In other words, the moral rules followed by the agent will be imposed upon from without rather than by some inner sense of morality which rests upon the agent’s own values.
Animals and children know only this kind of ethics, because they are not mature enough. They are taught what to do and not to do in order to receive rewards and avoid punishments given by some all-powerful authority for an unknown reason, and for them ethics is just that.
It seems that we could interpret theistic ethics this way. There is an arbitrary set of rules given by an all-powerful deity, and in order to reach heaven and avoid hell, one has to follow them and apologize for his sins.
The set of rules is conceived of as arbitrary because most people do not seen how they are rationally grounded.
Therefore, it is important to seek a rational justification for the theistic ethics; otherwise the opponent has the right to reproach such an ethics for being heteronomous, therefore childish and unworthy of a rational agent.
The ethics come from without, in that the specific decrees are given from another Source. However, we follow those rules because of a sense of love within ourselves. I hope that’s a good starting place.
If there is a conflict between rational demands and love for God, one cannot be in peace with himself. Unless he stifle in himself the light of reason, but in so doing, he debases himself.
Western religion is not fully based on reason, although we expect it to coordinate with reason. It seems to be that since it is built on Faith, it makes sense that actions that prove Faith should be required (not eating pork, for Jews and Muslims). These are like a parent giving children rules they cannot understnad, or perhaps it is just their gift of fidelity to him. But what is important to see here is that that does not make following the rules irrational in the sense of violating the rule of reason, although it may ask for a suspense as to the cause of the unknown.