Religious is as necessary to some as immunisation is to others. It's as necessary to some as breathing is, as eating is, as being a vegan/vegetarian is. It's as necessary to some people as sleeping.
Some people believe all of these things to be unnecessary. Some people don't. Whether you believe it's necessary or not is completely irrelevant, I'm afraid.
I'll be the first one to admit I don't understand circumcision for aesthetic reasons, but I DO understand it for religious ones.
The parents are doing what they believe is right to protect their son from Gods wrath and displeasure, so protecting his immortal soul. Whether YOU believe in these things has absolutely no bearing on them or their family what-so-ever. For some, his immortal soul is actually more important than his physical one. Physical ones will end, they will die and expire. Then they will either be burnt or they will be put into the ground and be eaten by worms and rot.
And after then is where you are judged by God, on whether you go to heaven or hell or not. And as the Jews follow the Old Testament, theirs is a wrathful, fire and brimstoney God, so that fear is real and tangible.
As I said, the physical part of raising a child is just a small part of their health and well being, and these parents were doing what they believed was the right thing to do to preserve their son in the eyes of God. The judgement of men is a negligible one when it comes up against that.