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@FLRW
The most simple reason why somebody might not believe in a god is that he or she doesn't see any persuasive reason or evidence to believe. They don't feel the need to believe in a god to explain the world around them. Nor do they believe a god is necessary for human beings to lead good, happy, and meaningful lives.
Why are you wrapping your statements in strawman arguments. You accurately describe the atheistic position as a position where the atheist doesn't see persuasive evidence. That's fine and it reflects my views from when I was an atheist, but lets go to the strawmans.
They don't feel the need to believe in a god to explain the world around them
Argument: Theists believe in God to help them explain the world
There is a lot of reasons to believe but barely anyone is worshipping the God of the Gaps. We are not in 200 BC and wondering who is pulling the sun up every morning
Nor do they believe a god is necessary for human beings to lead good, happy, and meaningful lives.
Argument: God is a crutch to lead a happy, good meaningful life.
hidden argument. It is possible to live that type of life without God
the reality. You can't lead a good life, with or without God. You benefit off of the violence that keeps society safe, you likely eat meat that harms animals, you likely wear clothes and most clothing factories have very horrible working conditions involving almost slavery but certainly child labor. You have a carbon footprint. Your food, gas and living condition is affordable due to exploitation of the less fortunate. If you think you are capable of living a good life, you are wrong. Jesus is there to forgive your evil life not to help you lead a good one.
Life is meaningless for an atheist. It ends there is no afterlife. You die and this means that nothing you do matters. You will just be a grain of sand in history, you and the people you love will be forgotten. SO good luck living a meaningful life as an atheist.
Maybe as an atheist you can achieve happiness, but who cares. It's meaningless and it is at the expense of others. ENjoy your herpes from your atheist orgies, enjoy your popcorn lungs from vaping marijuana and enjoy jacking off to women exploited for their pussy and ass . and enjoy the animals you eat who ssuffered to get on your plate. Enjoy your life of benefitting off of the violence of police officers who keep society safe enjoy the cheap clothing you buy that was made by child slaves. If you can be happy in that type of life, don't pretend it is because you are a good person.