Life-long Christians who lose their religion almost invariably go through a phase where they are not responsible atheists, but callous nihilists. I go through this periodically myself as I'm dealing with my current confusion over religion, and it can get frightening.
I never used a God as my moral compass (though when I was younger I believed in one vaguely) but in an intro to philosophy class that really opened my eyes (in more ways than one) the prof showed us what she called the Two Pronged Argument regarding God-given morality:
Prong 1) God says "I'm good, thus everything I do/say is good." In this prong, things are good only because God says they are, so he's stacking the deck as it were and defining himself as good. He could just as easily have told us rape and murder is good, so it's all arbitrary.
Prong 2) God says "This is good, so I will command it." This takes morality out of God's hands, and makes it something objective, and thus, we can find it for ourselves without God.
Both of these are the only logical possibilities when a God gives moral commands, and neither of them are scenario actual believers want. They want a sort of middle ground where God is objectively good, but we need him to tell us what to do.
The fact that believers of different religions and nonbelievers alike can behave morally or immorally in generally equal number, it seems to be human controlled more than God controlled. My compass has always been myself and the people I know rather than some book or a vague being I couldn't even see.