That is something I have always said. Atheists borrow their morals from the Christian worldview.
If atheists came across a culture were rape was required for males to "move up in the ranks" of their society, an atheist can't tell them to stop. Different society, different rules.
Christians can tell them to stop because they are not following God's Word. If somebody wanted to rob my house, I would have no choice but to let them if I were an atheist. I would have to sit there and watch them rob my house and shake their hand before they left and say, "Keep living with your own morals, friend."