I think itās for various reasons. Opposed Iāll explain, vehemently Iāll need a dictionary for - my mother language is Norweigan. ;p
I grew up as a protestant christian, a rather devoted one at times as well. I worked for a year as a deacon/layreader for my local church and defended my faith valiantly against my āaggressively atheistā friends. While growing up, I started accepting the fact that the holes in my religion were growing larger and larger. I had always accepted evolution, itās logical, evidence based and it makes perfect sense to me. I had also always been interested in knowing things, when I was a kid my parents would buy me a lot of childrenās books on things like animal life, plants, the body and itās organs and so on.
The more I looked at the holes, the more I realised that this really wasnāt going to work. I kept defending my faith against atheist views though, but I more and more found myself agreeing with them more than I disagreed. They made sense.
I would often sit up at night contemplating the ābig questionsā in life. How does it feel to have eternal life? I donāt want to live forever, but I donāt want to die either. This would eventually spiral down in a huge number of questions my brain couldnāt find a logical answer to, and I would just have to surpress the thoughts to get some sleep. I prayed, but it didnāt seem to help.
I work at a bookstore. One day I saw the book God Delusion by what I would reckon is the forumās favourite author, Richard Dawkins. Before, I was afraid that challenging my religious beliefs would be something really difficult, and would turn into an internal struggle I had to have the strength to withstand. I finished the book, and didnāt feel changed at all. Am I an atheist now?
Soon I started realising how obvious things were. Itās sort of like watching life through a pair of glasses with waaay too much strength, and then finally taking them off. Iām not going to live forever. Iām going to die, perhaps tomorrow, perhaps in 60 years. Thereās nothing I can do about it. No big questions.
The feeling you get when you realise how amazingly beautiful this world is, the complexity, the elegance, the wonderous results of millions, billions of years of nature doing itās best to survive. Itās so graceful. Some people describe it as a religious feeling, from personal experience I would say itās even better.
Why am I opposed to christian faith? Itās sort of how a catholic may feel when they contemplate the beauty of god, and is filled with joy and really just wants to tell everyone about it and show them how he feels - let them see the world from his point of view. I used to look at the world with christian glasses because I didnāt know I could take them off. Now I did, and I just want to show everyone how beautiful it is.
In part it is also because I am worried about the important issues being decided simply because āgod said soā, or well, simply for religious reasons. Abortion, gay marriage, war, peace, attitude towards others, morals, laws, all these are things which should very much be debated both for and against, but not with religion. Iām worried that schools will teach creationism instead of evolution, and Iām worried for all the kids who are taught what to think instead of how to think.
Basically.
TL;DR: Various reasons.