Most esteemed sir,
Consider my following argument against the thesis that God exists.
http://www.typologycentral.com/forums/philosophy-spirituality/8404-why-i-do-not-believe-god.html
Points to address.
Dostoesky's Grand Inquisitor was intended as a polemic against the religious corruption and beauracracy of the Roman Catholic Church. He meant to point out that they were following not the laws of God, but the laws of their religious teachers. Most prominently the Pope and the officials of the church.
You certainly can use this piece to motivate an argument in favor of the Divine Command Thesis. Or as Ivan Karamazov of Dostoevsky puts it, 'if God is dead, all things are permissible.'
The Divine Command Thesis is as follows; God is the origin of all morality. Hence, God's might makes right. Murder is wrong only because God said so, if he were to decree murder as a virtue, it would no longer be wrong.
Morality is a term that is synonymous with ethics. It is a study of desirable thoughts and actions. (See this paper for further inquiry into the nature of ethics,
http://www.typologycentral.com/forums/philosophy-spirituality/12093-nature-ethics.html)
If you are a proponent of the Divine Command Thesis as Ivan Karamazov is, you would argue that whatever God desires is desirable.
The question to follow is why that is the case. One may argue that this is the case because God always knows what is good for us and God is benevolent. Therefore God can only desire what is desirable for us. This is an unacceptable explanation for an advocat of the Divine Command Thesis because this shows that what is morally good exists as a thing in itself. The fiat of God does not decide what is good. God is merely the communicator of morality or the message that shall place us on the path conducive to desirable results.
In conclusion, the Divine Command Thesis is not a morally justified action because it is completely irrelevant to what is desirable for mankind. In condensed form, to say that someone desires something has nothing at all to do with what is desirable for mankind.
The deplorable behavior of the Grand Inquisitor and his followers is a result of immorality, or people having failed to do things that conduce to their long term well being. This problem would be corrected only in the event of them being informed of what is ethically justified. Uncritically accepting the dictates from another character or embracing the Divine Command Thesis does not entail an achievement of such an objective.
Earlier I have stated that Dostoevsky's Grand Inquisitor story could be used to motivate an argument in favor of the Divine Command Thesis. The logical soundness of an argument resultant of this endeavor would be merely apparent and not genuine.
It may urged that the Roman Catholic Church engaged in wicked practices because they have disobeyed the dictates of God. Lets assume, for the sake of the argument that the Roman Catholic Church would become more obedient to the ways of God than it has been before. In this regard the Roman Catholic Church would be much more similar to the Protestant Church. In other words, they would obey a set of dictates with regard to how they ought to behave more closely. It it still highly likely that they would engage in evil practices or those that hurt mankind. That is because the commands in the Bible cannot provide the adequate ways to handle all situations we may encounter. There is a myriad of situations that we could encounter, and a very small quanity of prescriptions with regard to how such situations should be handled. Therefore we are unlikely to find a one to one match between the entities of the former group and the entities of the latter. This shows that the Divine Command Thesis cannot provide instructions with regard to how all situations are to be approached. Secondly, it is not at all a guarantee that the word of God offers desirable instructions with regard to situations it does address, as we have established that what is desired by one character in itself is irrelevant to what is desirable by mankind.
Hence the Divine Command Thesis is untenable for two reasons. The ethical instructions it offers often contain errors and such instructions do not address all issues that need to be addressed.
Your most humble and sincere admirer,
SW