Is it true that Christians would be immoral without the Bible?

One atheist put it this way, “I’m terrified by theists who would be murders and rapists if they didn’t believe in their sky-daddy’s eternal torture dungeon.”

If you look past his obvious loaded language, his retort demonstrates either confusion over what it means to have a moral standard, or at best, a deflection of the idea that non-believers’ morality is nothing more than a matter of preference.

The Bible gives Christians a standard by which one may justify calling something good or bad.  If the Bible is of God, then its pages give us justification to call something moral or immoral.  But, playing devil’s advocate, if the Bible is not from God, then Christians and Atheists alike have only personal preference as morality, and that’s the point.  There is no true morality without God, only preference.

When a non-believer asks why Christians need the Bible to know that murder is wrong, they presuppose that we wouldn’t know without having a belief in God.  I don’t know any Christian who believes that. Romans tells us God’s law is written on our heart. This gives Christians reason to believe that everyone, atheist and Christians, know it is wrong even without having read a single sentence of any book of the Bible.  But by what standard does one use to justify it?  God – or personal preference?  Are these not the only two options?