I grew up in a country where religion class is mandatory. But we are not talking about having a class in Christianity or anything like that. It did take up Christianity more than the other religions, but this was during the time of a state church, and it can therefore be useful to know the biblical stories. But mostly it took up the various beliefs of various religions.
I have no doubt that this is a good thing, and an effective tool to stop fanatical religious indoctrination. It shows the kids that there are other options available. I am therefore definitely in favour of mandatory education in religion.