In my defense of secularism so far - the proposition that religious reasons ought not to be used in making policy decisions - I have used arguments that the vast majority of theists should also agree with.
They know the merits of prohibiting religous arguments from the courtroom. It would bring into the court as "evidence" any number of claims of the sort, "I have talked to God and he said the accused is guilty, so the accused is guilty." plaintiffs and defendents to pay for the services of those who are best capable of making a convincing case that "God told me that my client should win this case."
The same arguments apply to the use of religious reasons in matters of policy. It fills policy debates with all sorts of baseless claims. It also corrupts religion as those seeking to get their policy enacted shops around for priests who will tell their followers that God favors the person who has purchased the preacher's testimony.
You do not have to be an atheist to see the merit in both of these objections to religous arguments on matters of policy.
Yet, there is a third argument.
There is no God, and all religious claims are founded on a false premise. God favors no side in policy debates because there is no God.
As a matter of fact, what is really going on when a person makes a religous defense of a policy decision is the agent is simply giving his or her preferences. "There are certain things that I want. However, they involve creating states that harm others. I doubt very many people will go along with the idea of harming others just so that I can get what I want. Therefore, I am going to claim that it is not ME who wants these things, but God. Furthermore, I will claim that God will punish us all of He does not get what I want . . . um . . . I mean . . . what God wants."
I will not deny that the people making these claims often believe that they are serving some diety and that they are reporting what God wants. It is not the case that they are all liars.
However, it is the case that they are all mistaken.
People are known to be quite good, not only at lying to others when it serves their purpose to do so, but also at convincing themselves of convenient fictions. An individual who wants to do something that is harmful to others will often justify the act in his own mind by convincing himself of falsehoods that give the act some sense of legitimacy.
Child molesters will convince themselves that sex with a child risks no harm or provides a child with a benefit. Rapists convince themselves that women actually like being raped or that they deserve rape as retribution for some crime such as teasing or mocking men. Insurance companies deserve to be ripped off, and it is permissible to shoplift because the company is ripping off its customers anyway. My company is not treating me fairly, so it is an act of justice for me to take this money from the cash register.
Or, what amounts to the same sort of reasoning, "God says that I can go ahead and have these things that I want even though others are harmed, because God says that those others are an abomination - sinners - heathens - violators of His commandments and thus deserve the harms that they will be caused to suffer."
Secularism is justified because every attempt to justify policies that cause harm to others using religious arguments fit this model.
They are all examples of doing harm for no good reason.