Religious people like to say that religion provides an objective basis for morality and values and such, whereas without religion these can’t be anything more than just opinion. Of course, even if this claim were true, it would prove nothing about the existence of God or about the goodness (in any sense that is meaningful to human beings) of any values that God might prescribe. It’s no argument to say that something must be true because it would be bad if it weren’t.
But the main point here is not about why the argument is false, but rather about why it’s persuasive (I assume it’s persuasive because I hear religious people making it all the time). I think the reason is this. Suppose Doubting Thomas tries hard to figure out the truth about these matters, and has some not completely conclusive and only partially satisfying answers to show for his efforts, and he’s had little fun along the way.* His buddy Pious Pete points out that Thomas did a lot of struggling and still doesn’t even claim to have a rock-solid foundation for his values whereas he (Pete) does. That is, Pete correctly points out that Thomas faced a hard problem, struggled, and still didn’t find a fully satisfactory solution, while Pete went through none of this. Of course Pete didn’t solve the problem either, he just avoided it by refusing to take it seriously and essentially assuming it away. But it sure looks like Pete not only has the better deal, but is also righter; after all, he dealt with a lot less muss and a lot less fuss than Thomas did. Misdirection, just like a magician, and it works.
*I think it’s fair to say that most people who either don’t believe in God or don’t accept God’s values don’t believe that values are just opinion (some of my views on this are in the comments to this post). But some do, and even those who don’t probably had to do some real work and give up some cherished beliefs to get to whatever resolution they arrived at.