People are bound to disagree with one another when it comes to right and wrong. The argument in question says that morals can only be objective if there is a God. If you throw God out of the picture, moral choices become only a matter of feeling, and there's no standard to decide if one person's feelings are better than another's. If there is no standard, every moral choice becomes permissible. That is, everywhere you go, no matter what the culture, there are certain fundamental values that everyone agrees on.
Loyalty is Good.
Treachery is Bad. Loving others is good. Selfishness is bad. Yes, there are individuals out there who do not agree with these codes, but they are the exception to the rule. In other words, people seem to have a moral code written on their hearts.