My own personal theory is that religion most likely began because people were reacting to events of nature which had the power of life and death over them. Thousands of years ago their level of knowledge had not developed far enough for them to be able to understand how these, often devastating, events of nature occurred or what to do about them.
Once they decided such events were caused by some kind of mystical spirit the only thing they could think of to do was effectively suck up to them and hope they could persuade the spirits to be nice to them. Nature, of course, not being a sentient being, carried on as normal and the people then thought perhaps they had done something wrong and tried ever harder to keep these spirit gods happy with them. Over time, it bacame more and more unacceptable to upset the gods.
Nowadays, we are far more able to cope with events of nature and find ways to protect ourselves and overcome the worst of the consequences of most things. When we need a loaf of bread we can just go into any of dozens of shops and it's always there. In those days’ floods, storms, droughts, plagues etc could wipe out all their crops, wipe out all their livestock and even kill entire villages and they couldn't do a thing to stop it.
Having established that upsetting the gods was absolutely not accepted it would almost certainly have been a capital offence to upset the gods and bring down another major natural disaster.
Through the ages religion has built on this over thousands of years and come up with endless control tactics to keep people in line and ensure they continue to be "good" faithful believers. Religious leaders have further honed these skills for various purposes to keep people in line so, even today, it is still seen as highly unacceptable to offend religions or their made up gods.
Hence, anyone who thinks differently must be some kind of militant extremist.
Considering we all know that religious violence and the prejudice created by their out of date rule book has probably killed millions over the millenia and yet I have never, ever heard of a single case of an athiest ever harming a single person so this jibe that we are somehow the militants is quite frankly laughable.