There is no objective morality. Morality has changed with each culture. For example, slavery, which was condoned by the Bible, has been ended. Inequality of women and homosexuals, condoned in the Bible, has been ended.
I pose that there is [u]no right or wrong with the existence of a god[/u]. Divine command theory states that whatever god proclaims is right is right, and whatever is wrong is wrong. For example, when god told the Israelites to slaughter every man, woman, child, and animal of the Amalekites (1 Samuel 15:3), you cannot say this is wrong while believing in god.
However, secularists can say that slaughtering children is wrong. There can ONLY be morality without a god. Morals change, and the only way to improve upon morality is with secularism.
But you seem to misunderstand the Christian concept of good.
However, you're absolutely correct on one account. If "good" is only good because God commands it, it really isn't good at all. It would simply be subjectivity on a higher level.
However, Christianity believes that objective good in the universe is not created or invented. It is inherently tied to the nature of God, eternal and uncreated. God did not invent "good," He simply [i]is[/i] good.