Has anyone ever had the following discussion before? You assert that "if God does not declare something to be wrong then it isn't wrong" only for an atheist to assert that they don't need God to think that thing to be evil. How? If God is not the source of all morality, then who is? Is it the mere opinion of the atheist himself? Is it government? Is it culture? None of these make any sense when examined closely.
When someone makes this assertion — namely, the assertion that someone can believe that an evil thing is evil without needing God to tell them — then they are changing the subject. The original subject was the existence of good and evil generally. The subject that the atheist changed it to was his or her personal beliefs about morality. Any change in the subject, no matter how subtle, in attempt to justify one's position is called the red herring fallacy. Oh, wait, is it just evil for you but may be good for someone else? Then why are you judging it?
There is a simple way to refute the assertion that all morality is opinion and belief. Simply ask the question "Is that morality a mere opinion or belief?" It's a moral claim, is it not? Then how can that morality be absolute without contradicting the very standard that it is supposed to convey? Moral relativism is a self-refuting idea. It, like truth denial ("is that true?"), denial of the existence of absolutes generally ("is that an absolute?"), and knowledge denial ("do you know that you cannot know anything?"), is a violation of the law of noncontradiction and, thus, false on its own merits.
Appeals to law to determine morality don't fare much better than the above. If government were the source of morality, then every conflict would just be one government's moral opinion against another's. Would the Nuremberg Trials have had any valid basis to them if this were the case? No, and in fact the people making the arguments at the Nuremberg Trials had to admit this in order to give the guilty parties any sense of conviction. When it comes to morality, societal relativism — appeals to society as the source of all morality; society depends on government to keep it from collapsing — is a bad basis.
Cultural relativism — the idea that morality is a cultural construct — is just like the above. Those atrocities committed by ISIS — are they evil? I certainly believe that they are, and so do most atheists that I have debated. Unlike an atheist, however, I know where to properly ground that belief. Islamic culture is a culture, that much is known, and the rationale that ISIS uses is cultural rationale. In order to declare beheading to be wrong, one must go beyond cultural relativism as well in order to judge it as such, because cultural relativism definitely doesn't give an atheist grounds to tell a Muslim that beheading is wrong.
So, moral relativism — fail; societal relativism — fail; cultural relativism — fail. Relativism in general is always inherently self-contradictory, yet the only possible explanation for morality if there is no God is some flavor of relativism. Either relativism is true or God exists. Relativism is self-refuting and therefore false; thus, the only option is that God must exist.