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. This law is still there in a people even if they abandon faith in God. And because the code is so similar universally, it seems it must be objective. So far so good.