This is an argument I have had with a few theists.
When you read the bible, you know parts of it are moral and parts of it are immoral. How do you decide which parts are moral? Isn't that ability to identify the moral parts the real source of your morality?
I would ask them, "What moral standard did you use in deciding which religion was the morally right religion to follow?"
I think it fair to say that most people never really did research religions to find which was morally acceptable and which was not, they just accepted what they were taught without question.
The situation is, A) Either you accept the religion taught at birth, B) you apply some moral code that is independent of region to decide which religion, if any, is morally justified or C) you assume one religion is true and research all other religions based on that assumption.
For a while, I was asking everyone I could to explain why they believed what they did. I wasn't even trying to challenge their answers; I didn't care if I thought it was crazy or not. I just couldn't believe how hard it was for people to explain why they believed something so important that they based their lives on it.
The first idea that came along just sort of stuck, and they never considered the possibility of seriously re-evaluating.
In the moral dimension, I find it very common for people to be stuck in the stages of "something is wrong because I'll get in trouble" or "something is wrong because it's against the rules" without every moving on to the stage of considering the morality of a thing because of its consequences or intrinsic value.