Heres what I always thought about atheist:
What is so hard to believe that there is someone who cares about you? To believe that there is eternal life with nothing to fear or cry about. To believe that someone died for you and loves you so much. To believe someone can save you and defeat all your enemies. To believe that someone will do anything for you, as long as you believe in him and trust him.
Care to answer why they don't believe?
Hello. As a former Christian, I can help shed some light on your question "Why are atheists atheists".
But first, let me ask you a question of my own: Why are atoothfairists atoothfairists? What is so hard to believe that there is someone who cares about you and your teeth? To believe that there is truly a supernatural entity that is willing to collect all your baby teeth and give monetary compensation? I think we can all agree that the toothfairy is a marvelous idea. As an atoothfairist myself, I will admit that the world would be a better place if the toothfairy existed. I also think the world would be a better place with renewable energies, sustainable food systems, and cures for all natural diseases.
Let me make one crucial point here: Belief is NOT a choice. No matter how hard I try, I could not bring myself to believing the toothfairy, even if I am cognizant of all the potential benefits it could bring me. Similarly, I think it would be great to be able to live for eternity in complete bliss and happiness. But believing in something doesn't make it true, and dealing with reality on reality's terms is the best way (for me personally) to maximize is the best way to maximize your contribution to humanity, your life experience, and your happiness in this life; the only life that we truly are guaranteed to live

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Now obviously I can't speak for all non believers because not everyone rejects the claim of a God for the same reason. There are some people that are born into completely secular societies, and receive education about many different religions as a youngster, and develope objective opinions on God(s)/ religion via latter research and rational inquiry. There are others who are influenced by personal experiences, and others who are highly humanistic by nature. There are also some who choose to stop believing due to negative experiences they had within the church.
Like I said I before, I was a Christian for 16 years. I spent a large amount of time learning about the history of the religion and the bible. I studied old manuscripts and even personality translated sections of the Dead Sea scrolls and Greek manuscripts to Latin. I read many books on Theology and Christian Apologetic. I studied biology and genetics in great detail. And after years and years of research, I realized that I personally just could not believe in the Bible anymore, nor could I accept Christian dogma/ doctrines as a whole.
I called myself a deist for many years after as I continued my research. I started to realize that I no longer had any reasons to hold onto my belief of a God/ deities. I started calling myself an atheist in 2011, however I do not recall when I actually stopped beleving in a diety. I am currently a PHD student at UNC Chapel Hill focussing on early Christian manuscripts and Greek Manuscripts from the 8th & 9th Centuries.
Please feel free to shoot me some questions if you have any
Ethan