The Delusion of Sameness

As I have posted before, the religious divide in our family is fairly great. My MIL and FIL are very Lutheran, very churchy, very prayery, and very "God is Love"y. I, however am the polar opposite of this. I believe that everyone is connected, but that "God" does not exist. At least not, God, the Father; or God, the Almighty Ruler of the Universe who plays with us as if we were Barbie dolls; and certainly not the literal GOD as written in the Old Testament. My DH is kind of in the middle, leaning toward my way of thinking but with lingering religious guilt as perpetrated by the strong beliefs of his parents and his upbringing therein.
Of course, we have never actually DISCUSSED these beliefs with my MIL and FIL because my MIL would rather not. She would rather not discuss anything that would cause a confrontation. She is happy to just go on believing with all her heart that we believe what she believes. It seems to make her happy to live with this delusion. If we don't talk about it, it can't be true.
This delusion is now being tested as my son grows up. He is now six years old and brutally honest. He has no tact. He has no diplomacy. So when my MIL asks him, "Do you sing Jesus Loves Me at church?" (We are Unitarians.) He says matter-of-factly, "We never sing about Jesus."
Now, I think that this would be a WONDERFUL opportunity to discuss something. If it were me, I would say, "Oh really? Why not? Does your church not believe that Jesus is the son of God?" especially if I did believe that. But not MIL. She gets as far as "Oh really?" and then clams up. She is probably thinking, "Forget, forget, forget!!!!!!!" There have been similar lost opportunities over the past couple of years, most specifically when the in laws actually attended church with us. I truly believe that if she wasn't so afraid to talk about our differences, I would be much closer to her. But again, it is all too uncomfortable for her.
My friends keep reminding me that I would be so unhappy with the lack of results if I were to try to get the conversation started. They point out that she is not like me, and that I can't change her any more than she can change me.
So this Sunday when my in-laws called, my son announced, "I'm going to brush my teeth and go to bed." My MIL responded with, "Don't forget to say your prayers." While this would have been a great time for my son to say something like, "Oh we talked about prayer today in Sunday school. We learned that everyone prays in a different way," (which is true; they did!) our little man said in his annoyed, snotty voice. "We don't do that in this house," and hung up the phone.
I was more annoyed with his tone than with what he said, but my DH seemed a little more upset. Is he feeling guilty and that his Mom might be disappointed in him as a father? Maybe a little, which in turn makes me angry at her.
The good news is that the in-laws are coming in a couple of weeks for a couple of weeks. Now that they are retired, they can come and go as they please. (yay!) Wouldn't it be great if during this visit we really hashed it all out and put an end to this passive aggressive religious non-discussion?
I keep believing with my whole heart that we'll actually talk about it some day, and that she will understand me. I guess that is my delusion.