Each of us demonstrate that we believe in a higher law of morality - something higher than our own opinions and preferences - something objective, beyond ourselves.
Atheists will typically respond that what we call 'right' and 'wrong' are merely cultural norms or societal conventions that have evolved over time. Subjective ideas of right and wrong that have come from the individual and which the group has agreed upon. They assert that the differences in morality from culture to culture and time to time proves the point.
But whenever you find a man who says he does not believe in a real 'right' or 'wrong' you will find that same man going back on this a moment later. He may break his promise to you, but if you try breaking one to him, he will be complaining 'It's not fair, you promised.' Does this not demonstrate that all people regardless of time and culture believe in an objective moral law? Has there ever been a culture where breaking a promise was a virtue?
You might think you are justified in breaking a promise or that you haven't really broken a promise at all, but when the tables are turned you realise that your reasons don't hold water. Both parties know that it is objectively wrong to break a promise. The quarrel is not about the rightness or wrongness of breaking promises but about whether this law has been broken and whether the offender was justified in doing so.
From 'The Godless Delusion' by Patrick Madrid and Kenneth Hensley