Most people who discuss the moral argument for God’s existence make a basic mistake: they form the argument based in part on the idea that everyone has a consistent idea of what morality is. That is not necessary.
I don’t think the standard form of the argument, that everyone's morality is the same when they express it, makes sense. That form ignores the reality of original sin or total depravity. Briefly, total depravity is not that all people are as bad as they possibly could be. It’s the idea that all people have a nature that is prone to disobey God, and that nature effects every part of their lives. No one’s conscience is immune from this, so no one’s conscience is perfectly conformed to God’s law.
But the argument doesn’t have to show that everyone’s morality is the same as they express it. The form of the moral argument I use comes from the Apostle Paul:
“Therefore you have no excuse, O man, every one of you who judges. For in passing judgment on another you condemn yourself, because you, the judge, practice the very same things. We know that the judgment of God rightly falls on those who practice such things.” (Romans 2:1-2, ESV)
It’s not that everyone knows the moral law off the top of their heads. It’s that everyone knows what is wrong when that wrong is done to them.
Two examples to make a point:
Let’s pretend for a minute. It’s 1930s German. You are a Jew. I am a Nazi. I have a gun pointed at your head in an attempt to exterminate you and your race. My culture is telling me that I should kill you. What do you say? Should I just feel sorry for you? Why should I empathize with a person who has no value and not pull the trigger? My culture and my sensibilities tell me to destroy you. What do you say? (Adapted from a lecture given by Walter Martin)
Another short but pithy comment overheard by Ravi Zacharias: “In my culture it is taught that we should love our neighbor. In other cultures it is taught that we should eat our neighbor. Do you have a preference?” (wording is from memory of a lecture and not exact)
This is the best form of the argument. This one carries weight.