communion they literally believe they are eating Christ and drinking his blood through transubstantiation.
The most asinine custom is the practice of parading around a dead man on a stick to remind people that the died for their sins, when it is more like a macabre way of saying you have to suffer in life to get your reward after death.
Once you become a true atheist, all religious practices become completely asinine, but I think it has to be praying for me, as it is at the core of 'having a relationship' with god and so much useless time is spent doing it. Added to that are the stupid studies they quote saying that prayer actually works plus all of those FB posts saying 'pray for us' or 'thoughts and prayers'; it is endless amusement seeing anything about prayer for any religion. Prayer is more like schizophrenia than anything magical.
I was going to say it depends on the type of belief, but then I realized that belief in something for which there is no objective evidence in spite of ample evidence to the contrary is a pretty asinine thing to do. So, please forgive the sweeping generalization, but believing is the most asinine thing believers do. Everything else pales in comparison.
Wow, so many. I think it all can be called magical thinking. Bargaining with a god, thinking that you are a special creation while others suffer and die is selfish and irrational. The idea that a god is concerned about what you eat or your sexual practices has become completely foreign to me. It is something silly from childhood, as well as being damaging to human beings.