How can there be absolute morality if every person establishes their own definition of morality based on their own unique opinons and upbringing?
That is a good question. You are asking it rhetorically but it need not be rhetorical. A better way to phrase it might be, "How is absolute morality defined when morality seems to be derived on an individual basis?"
This question is based on the presupposition that morality is in fact absolute; otherwise you would have no valid arguement against the Bible.
To presume that an absolute morality must come from revelation is a fallacy that many religious people make. Who is anyone to say that such a morality comes from their particular ancient book?
If a nation expects to be ignorant and free, in a state of civilization, it expects what never was and never will be. --Thomas Jefferson