Some people think Christianity uses circular reasoning and/or relies on the principle of might makes right: Whatever God says is automatically right, because he's God and is defined to be good and/or he has the power to enforce his laws. However, the Bible states that God should be obeyed because he's perfectly just and righteous:
Your statutes are wonderful; therefore I obey them...The statutes you have laid down are righteous; they are fully trustworthy...Your promises have been thoroughly tested, and your servant loves them. (Ps 119:129, 138, 140)
It also states that some actions wouldn't be right even if performed by God:
"Will you sweep away the righteous with the wicked? What if there are fifty righteous people in the city? Will you really sweep it away and not spare the place for the sake of the fifty righteous people in it? Far be it from you to do such a thing - to kill the righteous with the wicked, treating the righteous and wicked alike. Far be it from you! Will not the Judge of all the earth do right?" (Gen 18:23-25)
Therefore, good is not circularly defined as "whatever God does, because God is always good." Even if morals were somehow created by God, God could not arbitrarily decide to change their definition such that an evil act would become good, or vice versa.
Other examples of actions that would be wrong if performed by God: